1. What should I practice with Snake as a daily thing things I can do by myself
Any and all tech-related things, honestly. If your inputs can be perfect (or as close to perfect as possible) then you have a lot less to worry about overall and can focus on the more nuanced aspects of your gameplay. A quick list: L-cancels, wavedashes, SHFFLs, dashdancing, wavelands, shield drops, aerial interrupts, glide tosses, aerial glide tosses, DACUS, b-reverse, b-turnaround, Cypher canceled Aerials, run cancels, pivots...
The cornerstone to good snake play (like any character) is being able to make the character do what you want them to do 100% of the time.
You can also practice simple combos like chaingrabs or nair strings where there isn't really much variation that can happen.
Practicing spacing and hitting shield with Snake's aerials to make them safe is also really good. Cypher cancel back air into dash back tranq is a possible whiff punishment on shield grab, for example.
2. Stuff I should do in matches like sticking and how/when (besides tranq to stick throw stuff) where I should place mines, combos with percent, etc..
Up throw stick is guaranteed at low percentages across the cast to varying degrees.
Crouch Cancel C4 is a solid option in some cases.
Cypher Cancel C4 works if you react to their DI.
SHFFL nair can combo into a C4 stick. Get the hitboxes really low to maximize your on-hit advantage.
SHFFL down air can combo into a C4 stick. This has a lot less reach than nair but is slightly easier overall.
Down throw is good once you condition them to wait instead of instantly rolling. C4 stick conditions them to instantly roll.
Techchases in general are nice for C4 sticks. Any instance where you'd get a grab you can probably C4 stick just as well.**
Being above Snake on platforms tend to make people wanna shield, which is when you jump up and C4 stick them.
You should basically c4 whenever you don't have one on your opponent and once you do you're already winning the stock, essentially. C4 stick is a huge win in the neutral for snake since it effectively halves all characters' survivability.
Mines are a bit more fluid and abstract a concept because it depends a lot on the character you're playing against and how the opponent deals with your mines. Dashdance heavy characters where I feel comfortable walling them out like Sonic would be good to sandwich between a mine and myself, effectively forcing them to stand still. Characters whose neutral depend on platforms like Samus's missile canceling get locked down by mines being on platforms. The most useful aspect of mines is that they force your opponent to play differently; the trick to utilizing mines is to understand how you can force these changes and why it can be advantageous. There's not a hard and fast answer to this.
Combos are weird but your major combo starters are stuff like Cypher, up throw, nair, dair, down tilt, grenade, and f-tilt. Combo extenders/juggle tools would be things like Cypher, up air, back air, nair, dair, down tilt, up tilt, and up smash. Essentially, weak hits can lead into other weak hits which can then lead into strong hits. Everything ends in C4. Or mines. Or up tilt. Or fair.
3. Neutral what moves to use and how
This again depends a lot upon the character you're playing against and changes depending on how the opponent is using their character in neutral. Typically, Snake isn't a character that can approach outright because of his lack of speed, agility, safe shield pressure, or hard pressure options. He's more equipped to utilize what movement he does have (dashdance, crouching, crawling, b-reverses, Cypher) in order to make an opponent want to stuff his "approach" so that he can then punish their attempt to hit Snake: whiff punishment. They miss a move and you hit them in their endlag. You can also apply soft pressure through grenades on the horizontal spectrum and Cypher and up smash on the vertical spectrum, but they're not what I would consider "hard pressure" like Fox's or Falco's lasers where they immediately threaten you for next to no commitment. I'll use Squirtle as an example: say he's dashdancing a ton and acting like he wants to come in, so I might pull a grenade and then approaching SH back air, knowing the back air will auto-cancel out of short hop if I do it quick enough. If Squirtle tries to hydroplane grab or down smash me, he risks getting hit by the grenade, and if he decided to jump at me, back air will trade or stuff anything he tries to throw out while still being relatively safe on whiff (because I can choose to SHFFL it at any time, or just have 4 frames of end-lag. I also know that grenade's explosion doesn't hit snake when he SH back airs if I'm at the peak of my jump when it explodes, so I can time my back air and grenade in unison to cover multiple approach options at once. Compare this to the Falcon matchup, where Falcon's main ways of starting combos are SHFFL aerials, auto-cancel up air, JC grab, or jab. All of these options lose in some form or other to Snake's crouch. If the Falcon knows this, I need to be creative with how I use my crouch, but if he runs in and goes for a JC grab he will whiff because he can't grab Snake's crouch with standing grab. SHFFL aerials can be shielded or stuffed out by up tilt depending on their timing (early loses to shield and late loses to up tilt).
Generally, safe neutral options would include spaced aerials like nair and back air, grenades and their tech aspects (AGT particularly), and spaced down-tilt. Anything else is likely to be dangerous.
4. What do I do when I can't set things up and am getting rushed down
Figure out why you're getting locked down and use the tools in your kit to stop that from happening. When someone's got the download on how to lock you down you've gotta change what you're doing. Adaptation is the word here. If I see that Pikachu's cross-up nair is beating my out of shield game: he's crossing up too far for me to Cypher or grab him and my back air doesn't reach him, he lands too fast for wavedash to be an option, and I'm just stuck trying to catch a faster character. Instead of fighting a losing battle, I've got to stop shielding and start dealing with Pikachu's nair in a different way. Maybe I short hop nair it. Maybe I back air. Maybe I go for a raw up-tilt read because he's C4'd at 80% and will die if I'm right. This is what I was talking about at the start with where your mind needs to be in a game instead of focusing on the tech stuff: if you can consciously think about why you're winning or losing a game
while you're playing the game, you'll be able to either exploit your advantage further or overcome your disadvantage more easily. This is
hard to do, but it comes with time. Keep playing
