Here's my edgeguarding flow chart in a nutshell:
1. If they're going high, create a situation in which it'll be difficult to land safely. This means covering space with mines, grenades, and mortars. If there are platforms, put a mine on the platform AND position yourself to make use of their reluctance to land there. Snake has amazing anti airs in Cypher, up air, up tilt, and up smash.
2. Continue this until they're unable to go high anymore, which means they're going low. Generally speaking, this is when you should be grabbing ledge, as that gives you the positioning you need to cover fast horizontal recoveries, gain invincibility, and allows you to ledgehog quickly. Alternatively, you could stand close to edge with the intent of wavedashing to ledge quickly, which gives you more flexibility if they decide to go low instead, of it it's a character like Fox who can still go high despite being low relative to the stage. There's lots of permutations here and it really depends a lot on what character you're edgeguarding.
3. With land mines, a huge mistake I see newer players make is to not synthesize these two aspects together. Many times in smash, a character's recovery can be made ambiguous, in a way that "just hold ledge" isn't enough and you need to do something more. That's where the mine at the ledge comes in, but if you yourself are not holding ledge that loses to sweetspot. So you need to hold ledge AND prevent recoveries that fall in the middle of your two zones. Basically, I'm telling you to wavedash to ledge after planting a mine. You have many options with invincibility frames to get through the mine which are not afforded to your opponent: ledge roll, ledge stand, waveland, and ledge jump all can detonate mines safely.
4. Now, a huge mistake I see more veteran but still inexperienced players make is that they place the mine and wavedash to ledge in times where it's not opportune, that is, when their opponent won't be KO'd by it. If they are below KO percentage from mine (or from the combination mine + c4) you shouldn't place a mine at the ledge and should instead seek to get a conversion off of their non-sweetspot recovery. This conversion might still be the mine, mind you; juggling is a very real skill that Snake has, but it's not always as good as simply resetting your edgeguard with a ledge stand back throw or a free ledge hop c4 or a free ledgehop tranq (Marth, Sheik, and Falcon all fall prey to this complex).
5. Finally, understand what tools you have to counteract certain recoveries. Back air is traditionally your best option as it has a large, active hitbox and a good trajectory for the occasion while also allowing you to regrab ledge after using it. Down air is another good option, as is Cypher. Fair has some situational usefulness that I want to explore more. Footstool is also something I've been experimenting with a lot more. Some recoveries have a Marth Killer susceptibility which is invaluable and changes how you coordinate your edgeguard resources.
Here is an example: I catch Marth with a back air on Battlefield at 70% and he gets sent far off stage because he misses the DI, but he's still high. He has a c4 on him, but it won't KO at this percentage. What I would do here is plant a mine on the side platform about 1/3 of the way in. This cuts off a lot of the high recovery options Marth would have. From here, I would set up the Marth killer by rolling toward ledge and read where his movement is taking him. if I see that he's coming in horizontally, I might SH back (the mine isn't close enough for it to bother me) to sniff out a bad side b or double jump that I could stuff with a back air, or I can actually tap back on my stick and fast fall to ledge due to how this positioning works. If I see that the Marth is going low to try and beat the Marth Killer, I'll set it up anyway and shield DI my way to ledge so that I can ledge hop up air Marth and KO with the C4. If I didn't have a C4 available here, I probably would focus much more on the back air since Marth only really gets one shot in this scenario. I would still set up the Marth killer regardless, but I might opt to try to ledgehop invincible back air his up b instead to secure the KO outright. Or I could opt to Marth killer the sweetspot attempt and then ledgehop c4 him if he managed to get on the stage, because Marth still has considerable landing lag on his up B. It depends a lot how the game is going and how comfortable I feel with that. The cruel thing here is that, without C4, Marth's best option is actually to go toward the mine and potentially get hit by it (he could possibly air dodge through it which is why we don't get to ledge right away; we still try to cover this with our SH or FH back air after rolling toward ledge). That's the essence of edgeguarding: cover all possible paths back to the stage with something that will either KO outright or set up for a KO.
Everything I described here requires quite a lot from the snake; knowledge of the opponent's recovery options, foresight for how and why certain options are good or bad for that character, knowledge of Snake's hitboxes and KO percentages, knowledge of Snake's movement, and the expectation of doing all of this simultaneously in a time-sensitive situation without flubbing any given inputs. That's a lot, but if you manage it, recovering against Snake is simply impossible.
