Assuming familiarity with basic Tekken, moving, poking, blocking and punishing with Yoshi is fundamentally the same as any other character. Players often dive into the deep part of his move pool without learning the basics and it makes for a harder time playing the character. He has numerous flashy moves, unblockables and animations that are very tempting to use but they come with properties that require a lot of knowledge and experience to be used effectively. Take it slow and first learn his basic moves, simple combos and general properties of his tools.
Where do I learn?
There are many learning resources all around. There are articles all over the net, video tutorials on Youtube or discussion threads such as Tekken Zaibatsu, Discord or Facebook. The best place to learn however, is in the community. Be it online or offline, play matches and get feedback. Playing and speaking with other players will be the best way to learn and get better.
Who is a good player to watch?
There are many notable players to learn from. Some of the current well known Yoshi players are Kari from Japan, Eyemusician from Korea, Kaneandtrench from the UK and JustFrameJames from the US. By nature, Yoshi's open play style creates distinct players so each individual plays noticeably differently from one another. There is enough diversity to give different perspectives and a good chance to find a similar or ideal play style to use as reference to get better. Copying other players is common practice but it is inevitable for each player to develop their own playstyle. Note that something that works for one player may not necessarily work for another so it is up to the player to find what works for them.
Does he require good execution?
Yoshimitsu's execution barrier is fairly easy. Majority of his move set have simple inputs and have a good buffer window for easy execution. Certain moves and combos require great timing but for the most part, his go to moves and combos can be done with minimal effort. Flashing Steel (cd+2), his new screw move, along with Gehosen (cd+1) are concerning moves for some due to the f,N,d/f input but these also have a decent buffer window so its timing is a little more forgiving then other crouch dash moves for other characters. All in all, it's easy to get by with average execution and the harder stuff should be achievable with more practice and experience.
How do I do this combo?
His bread and butter combo goes along the lines of a launcher, following a screw attack, body blow, right jab, then end with Oma Gehosen to Ganto ([launcher],[screw],d/f+1, 2, b+2,1~1+2 KIN f+2). Beginners often mistakenly use the d/f+1,2 string rather than using d/f+1 followed by 2. Some beginners also have problems doing KIN f+2 in b+2,1~1+2 KIN f+2 as one fluid motion. The string becomes second nature by buffering the KIN transition by holding 1 after b+2 then inputting the 2 resulting in b+2,1~1+2. There are many other combo formulas with varying difficulty and it's up to the player to choose which combos are practical, optimal and suitable for their game.
Do I go for damage or mix-up?
Desirably, having both damage and mix-up would be best. Realistically however it usually has to be one or the other. There is no one true answer as going for damage or mix-up is a decision of the moment. Players argue that Yoshi has low damage output so going for mix-ups would be best but on the other hand, always going for mix-ups is a risk and can be a waste of guaranteed damage. Choosing which route to take will be mainly dependant on the situation.
What do I use for wake-up?
He has numerous unblockables, full crouch mix-ups, stances or standing moves that are usable after combos or knockdowns. Though he there is no shortage of Okizeme (wake-up) options to choose from, they also come with varying degrees of effectiveness and risks. The amount of options can be overwhelming and choosing the right one from a wide selection can be a tiring exercise. It would be easier to lessen the load by starting with a fixed set of options and experimenting with additional ones later on.
What do I do against turtles?
Keep your composure. Opponents wary of Yoshi's shenanigans knows to back off and wait for their opportunity to counter and punish. This often causes players to go into auto-pilot attack mode and rush in to attack without much concern about the risks of their actions. Players tend to forget they also have the option to defend, step back or even heal themselves. It is best to be patient, mind the spacing and find more effective ways to take control of the match.
What do I do against rushdown?
Keep your composure. Opponents relentless with their attacks are often fishing for reactions they can counter and punish. Players often panic when they're getting rushed down and they eventually react in a way they reach the scenario of going from the frying pan into the fire. Flash or KIN parry is often used as a panic move but using moves in distress often nets bad results. Auto-pilot defense is just as bad as auto-pilot offense. There is no attack without holes so stay calm and collected, block, move and wait for the chance to counter and punish.
When do I use flash?
Flash is a double edged sword. Success can give great momentum and respite while failure can result in heavy counters or punishment. Use of flash will require good knowledge and instinct. It is best to start off learning which moves are flash punishable to be able to have a reliable usage of its lightning fast property. Learning how to interrupt opponents momentum for defense or using it after attacks for offense will come naturally after getting used to the character.
What does Soul Possession do?
Soul Siphon (qcf+2) is the throw that steals health from the opponent. Opponent breaking Soul Siphon will instead damage Yoshi and give opponent health. It's a risky throw but when you condition your opponent with breaking regular 1_2 throws, it's a good alternative as it is a 1+2 break. Soul Possession (qcbf+2) is a throw where Yoshi gives his opponent health. It is is essentially unbreakable as breaking the throw does not do the opposite as it still gives the opponent health. It doesn't give frame advantage or any special property to Yoshi. It's been in his move list since Tekken 4 and there hasn't been any known use for it.
What's that reversal move where he flips in the air?
Many players find this by accident but Kinchou stance (1+2) has an additional parry property when entered. The parry window is around frame 2 to 8. The follow-up move after the parry has the same animation as Guillotine Crow Kick (u+3+4) and a ground stab (u+1) is guaranteed on hit. Note that even after a successful parry, the follow-up isn't guaranteed. It gives plus frames on block but it can also be stepped. In other situations you can reappear behind the opponent with bad recovery that puts you in a very disadvantageous position. It's not very reliable with different possibilites but it does have its uses outside of flash range.
What are his secret moves?
Asking what are Yoshi's secret moves is a broad question. If people consider moves that aren't in the command list secret, then he has a lot. Some moves are more like extra properties of existing ones such as KIN parry. Some moves have extra inputs like shortened Sword Windmill or Sword Smash to IND stance transition. Alternative inputs can be considered by some to be secret like [4~1] for Flash or silent Sword Stab. He is also able to do certain moves in back turn and MED stance that isn't widely known. Some aren't actually secret like moves from No Sword Stance that people just aren't familiar with. There are definitely hidden moves with elaborate inputs and conditions like Poison Windmill and Poison Knee Taunt on Bryan. There are too many moves to name and possibly more that are still undiscovered.
When do I go No Sword Stance?
No Sword Stance has been around since Tekken 6 yet it's still not a household tool for many players. Going into stance requires intent and can't be smoothly transitioned into from common moves unlike other stances so many players are unfamiliar with it. The main feature of the stance is a substantial change in his standing game. He loses a couple stances, a few unblockables but he gains alternative moves and properties with a more solid feel for the character. It may not be intuitive to go into NSS but it's a great option against certain matchups or switching up the playstyle.
Is Yoshi top tier?
Tekken tier lists are based on player opinions about the current meta. They are often not definitive of the whole state of the game but they do have some credibility to them. Interestingly, Yoshi often falls in an unknown tier as most players can't fully define his capabilities. His common placing is usually in the average range being seen as neither very bad nor very good but some players would consider it to be dependant on the potential of the player.
Who are his best/worst match-ups?
Yoshi is a very good match up against big characters like Gigas. He's a large target for flash that greatly restricts his offense and the wide hitbox along with Jack-7 gives him strong character specific combos as well. His ability to flash punish key moves also gives him an edge against Noctis, Lucky Chloe or Master Raven. With the right spacing and defense, he is also effective against advancing characters like Steve or Lili. Bad matchups would be characters like Claudio and Lars who operate at the same range as him with better solid options. Yoshi's risky style can also be a problem against strong defensive and punishing characters like Kazuya or Feng. Every other character is debatable but it is important to note that each pairing is different depending on the players. Skill level, experience and playstyle are huge factors when it comes to match-ups.
How do I do the flashy stuff like kill with harakiri?
Majority of the people who pick Yoshi do so because he is flashy. It all looks cool but winning with a slow motion suicide kill isn't that easy just by playing the character. He takes a lot of skill to play and the reality is, it's actually hard to win with him. Players have to develop the fundamentals first then the highlights come after. There's a lot to learn and even more to figure out. First get a stable job and save the partying on the weekends.