Hey Sarah! If you wanna improve your edge guarding you first have to know your character and your opponents. It's better to focus on one or two characters to get better as the more you play with your mains the more you'll understand them. Now to actually talking about edge guarding.
Edge guarding is a very simple but risky concept.
By going off stage to attack your opponent you're also leaving yourself vunerable.
That's why you have to know the moveset of all the characters because some characters just have really amazing recoveries which means you'll have to approach them differently.
What works on one character might not work on another.
To practice edge guarding I advise going on training mode and putting the cpu's percentage around 100% and back throw them off the stage
Its not ideal since they dont mixup their recoveries but you get an idea of how to go off stage and still make it back.
Against actual people though, you need to predict what they'll do and when.
For example, characters who teleport I'll stay on the side of the ledge and use N.air to catch them.
Its just all about predicting choices, reading habits, and knowing the matchup.