Any attempt by Marth to fade back with an aerial can be punished with ftilt, and usually fsmash if you space it decently. Approaching you with an aerial opens him for shield grabs, or letting you run under AC nair. Most Marths love DD grab, so you should guide them to the corner or get close enough that you know you can hit before attacking. Otherwise if you want to approach with an aerial, get close enough that timing a fox trot back would be close to the point that your aerial would be hitting, either right at the start or right before. If they try to turn around you're probably very close to the royzone already, if they try to block without turning around then Marth's oos from behind isn't that great. Try using approaching shield (out of full run, NOT shield stop) to mitigate any attempt by Marth to approach with not grab (and time oos quickly). If you get in close enough, you can likely grab him out of a non-pivot grab attempt this way too, usmash has even better range. To reset and deal with the most common options like grab, you can do a retreating nair or wavedash back.
Generally, stay grounded and ready to shield. Don't be afraid to get in Marth's face since this can royzone most of his moves and running past him actually works pretty well. If you're far apart, laser, and don't run into his dd attempts. You can jump over dtilt, though he might start to opt to jab when this begins to happen, which actually becomes much more effective when he starts building percent on you. Stuff grab attempts. Always expect him to try to move around your attacks (DD, WD etc) or intercept them (dtilt, grab, or the occasional nair). If it looks like he's attacking at all, you should jab, grab, or shield.
Also get Marth above you. If available, careful circle stalling/drop through or run off platform is pretty safe, and the Marth player may try to intercept you. Alternatively, shield drop against that.
Run up jab is the safest approach most of the time, just don't get CCed shield grabbed or hit with the bizarre uair.
I expect you to know that having stage position is important, but you can always out camp other characters.