1. Dont get grabbed
2. Dont miss a tech
3. Dont get caught on a platform
4. Understand and apply combo DI for his Fthrow/Dthrow and Utilt
Overall its a difficult matchup. I find a lot of trouble with him as a character as well but you have to play extremely patient but capitalize hard when you get an open while at the same time, know your limits with Roy. Really though, play incredibly safe and don't rush your approach or get impatient. It sucks cause in early percent in neutral it can be dangerous to approach with Dtilt or Fair>Dtilt if he's crouch cancelling or anticipates the Fair with a shieldgrab. The best thing that i've found stagewise is to pick mid-larger stages like PS2 or FD so you have more room to work with and out grab-game him in early percents. I've also found that battlefield, warioware, and yoshi's are stages to stick away from cause he can juggle and catch you with Utilts too easily. My personal stage list preference against Marth is typically PS2, FD, FoD, GHZ, and then smashville isnt too bad either. Warioware and battlefield are debatable on this though because you also can do really well on either of those stages.
Unfortunately this MU in my experience comes down to who gets the first grab or who gets the early lead so you really have to be tight when it comes to clutching out Dtilts and grabs early on. Also another tip is to really ingrain DI'ing away when he tries to go for Fthrow chaingrabs or Fthrow>Fsmash. You have a great tech chase and combo game with a more consistant punish than him, I know Sethlon has said that the Roy v Marth MU is 50-50.