You get just outside of his tipper range, wait for him to use a move, then immediately punish. It's going to be either a mindgame or reaction battle, because blindly approaching marth will almost always result in a tipper. I like a campy mindset, since he doesn't have a projectile. Attackwise, you approach marth just like any other character, it's just the timing that you have to pay extra special attention to. Marths attacks have more lag afterwards than before, so watching and waiting for him to go first is when you approach with a shffl'd nair, grab, etc. Also, approach marth horizontally when he's on the ground. Marth's aerials are too good, but he can get lag from his attacks when he's on the ground. Marth is also pretty weak from below, but I've never had much success from approaching him from below, generally because his fair and bair reach us in ways our miniscule range doesn't. And uair doesn't lead into as many things as I'd like, at least not from that angle. Still, upsmash is good. Marth is weak from below, as any fox main can tell you.
Marth is a character where you want to be right next to him, or as far away as possible. Abuse your jolts when you're away from him, it will cause him to mess up more, and percent is always a plus. Once you get in, you keep up the pressure or combo. I like to use running nairs to get in quickly for a punish, but other options are using your shield and dodge options to let him get close (which isn't technically approaching), wavedashing and triangle jumps are good options on the ground, and (a personal favorite, but people pick up on it quickly) if you see him about to use an fsmash and you're just out of his range, you can techncially outspace his fsmash with your fsmash because his arm sticks out when he swings his sword.
So that's the idea, wait for him to attack you and punish, and remember to spam jolts. He should be approaching you more than you him.