If you're facing level 7 strikers my advice is to sweep the leg. If that doesn't work run for your life.
But I digress....
The most common usage of DI is survival DI. Basically just holding a direction as you incur hitlag (freeze frames) and the resulting hitstun as you fly off toward the kill zones.
You want to hold a direction perpendicular to the direction you are going to be knocked back, so when Fox up-smashes you hold left or right as you are hit and fly away. Most great players are prepping to use DI when hit or when they think they are about to be hit by a KO move. i.e. Fox's Upsmash is too fast to react to and can be dashed or WD into, so you will have to be preemptively thinking about using DI before you are hit by it. But most KO moves you can DI on reaction since you can see them coming.
Combo DI is also very common; same as survival DI but it's used to change which way you are hit when an opponent is trying to combo you (can also add in SDI). Most of the top 25 use this almost religiously in random directions, it's why you see them survive so long since they can't be comboed very easily.
SDI (and ASDI) are a whole 'nother thing and it can used to wall tech, Amsah tech, escape Fox/Falco's Up-Air, etc.