The most basic guaranteed set-up is dtilt into KO punch. This will work on anyone pretty much at low percent, which is usually around/
at their lowest possible KO percent. Lighter characters like Jigglypuff you will have trouble landing this on past 20% or so.
An other less easy method is to perfect shield a move (and they're close enough obv), which lets you act immediately afterwards.
You can bait a shield also, catch roll abuse if you know where they're going, or my personal favorite, predict where they're going on ledge get-up. From the ledge, they have 4 options:
-get up normally. full invincibility frames until they, usually, shield. and of course, KO beats shield.
-roll. if you are paying attention, you will know if they are inclined to roll onto the stage in which case you just face towards where they'll roll and KO them.
-get-up attack. Shield (perfectly in an ideal world) and KO.
-jump. this is what you don't want and is probably the safest for them. if they air dodge though, abuse their landing lag and KO.
and also waiting for laggy moves on their end and punishing with KO punch.
Also in general you having KO punch makes the person play very differently. They will try to run away and knock you away to hopefully make you lose your punch. Be aware that this is going to be their strategy. Some common ways to do this to a Mac are dash attacks, throws, and projectiles. adjust as necessary.