The best thing about Magnet and what makes it truly useful is there are so many situations where you can use it, which really makes it tough for the opponent to correctly DI your moves/combos. An easy place to start on where you CAN use it would be if you could Fair/DJC Fair someone, so when you're starting/continuing a horizontal combo you can substitute Magnet for Fair. Magnet is very good out of a dash as well as from short hops since it halts your movements and can create a persisting hitbox in front (or behind with a B-reverse) on the spot, this is great if you approach someone and their initial response is to shield.
If you use Magnet in the air, you simply jump to stop that Magnet and you can act pretty much immediately. From this point you could do your full jump for a rising/falling aerial or DJC to respond with a move immediately. If you start with a short hop the options are the same, but a full jump isn't as good, especially if you hit their shield with Magnet/are relatively close to them. If you DO hit their shield a good option from short hop Magnet is to jump and do another Magnet, at which point you simply float back to the ground the short amount of distance you're up. If Magnet connects you can either use your full hop to move towards them and continue your combo/pressure or you can throw PK Freeze to cover the distance created, OR if you're close enough to the ground you can jump and air dodge back onto the field and run over to them.
Magnet's tough to explain, even now I don't fully utilize it despite having played this character for so long, and Magnet is definitely something you have to get a feel for/figure out through actual combat as opposed to simple discussion/'theorycrafting. Just start to introduce it into your gameplay slowly and you'll find ways to use it. Also good is to hit the lab and to watch how other Lucas players utilize the move.