Alright, I’ll layout a simple game plan for Marth’s neutral. By no means will this be the be-all-and-end-all strategy for how to play Marth, but it will provide you with the basic building blocks. Marth’s goal in the neutral is scare the opponent into inaction, thereby letting Marth safely approach with his offense. The way Marth does this depends on his opponent.
Against characters with less reach and disjoint than Marth, such as Jigglypuff, you have to scare the opponent through walling in the mid-range. The mid-range is that spacing where you’re safe from normal attacks, but are still in danger of sudden burst options such as dash attacks/grabs and specials like Diddy’s monkey flip. Walling is the act of putting up large hitboxes to discourage sudden approaches. Marth’s main walling tools are pivot F-tilt and retreating short hop F-air/B-air for their wide coverage. At moments when you think your opponent will use a burst option, use one of Marth’s walls. If you guess right, the opponent will run head first into a barrier of sharpened steel. If you guess wrong, your attack will whiff, but the backward movement of your wall will likely keep you safe. Either way, a few repetitions of your wall will teach your opponent that he can’t blindly rush you down. Once your opponent starts waiting for the right opportunity to attack (I.E. after your wall), that’s when you’re safely free to get closer and use your main pressure tools (jab, D-tilt, N-air, grab).
So what about characters with greater reach than Marth, such as Rob with his laser eyes? Against these characters walling isn’t as useful, as the opponent can just shoot through Marth’s sword with an energy blast. Instead Marth uses dash-in shields and short hop airdodges in the mid-range whenever he suspects the other will fire their blaster. Guess right, and Marth now has the frame advantage to attack. Guess wrong, and Marth can just retreat to try again as his shield and airdodge have a low-commitment. Do this enough and the opponent will learn they can’t rapidly fire projectiles all willy-nilly, granting Marth the respect he needs to walk in and pressure them.
I hope this mini-guide helps. It’s the shortest I can make it without going into all the intricacies of footsies, yomi, and stage control.