https://www.youtube.com/watch?v=dwKA3Va3fiw
This is more general LS usage with a few edge guards in there somewhere.
As for ledge guarding, a few thoughts:
With LS up, run off ledge, fall a bit, then jump back to the stage. Don't always wait on stage.
Double jump near the ledge, or simply start it well before you go off stage with it, and pull LS at the top of your jump. That way if you fall below the ledge the LS will wear off before you SD and you can Rush up.
Mash jump a lot. You never know when the opportunity will present itself.
When going against hitbox recoveries(like Fox UpB) try to position yourself slightly to the side, so that your hurtbox doesn't get it but they get hit with a leaf. This can also lead to a footstool setup if you hold towards them and positioning is good.
If you go for the footstool, try to get it as low as possible. If you footstool too close to stage level, they can simply mash UpB. If they have an UpB like Mario's, where they usually want to be at a certain point to sweetspot the ledge with it, that's where you want to get them. It can be dangerous to go so low, but if you wait until the LS timer is almost out it's less risky.
Getting the footstool close the ledge can still cause them to react poorly and leave themselves open, but you are less likely to get a kill from it.
If you throw someone off the stage, pull out the LS asap so that it's still there when they get to the ledge, but it will soon wear off so you can use any move as a follow up.
If your standing at the ledge with LS up, be ready for a run grab on roll getup.
If your standing at the ledge with LS up, be ready to run off stage if they hold on to the ledge too long and run out of invincibility frames.
Mix it up. Going for it and not going for it. It forces them to react differently to getting back on stage which can leave them open to other ledge kill methods.
Play it safe. Don't get too greedy because it can easily backfire and kill you.
Not all kills need to come from footstools. Getting hit with a leaf can interrupt and lead to mistakes, forced SD's.
I don't think there are a many reliable setups. It varies greatly between recovery types, positions at the time, what move they might try to use against you. Some characters it's not worth it to really try too often. Multijumps and crazy recoveries like Lucario.