Fortunately there are already .rels for Marth over almost every character, here's a link to them:
https://www.dropbox.com/sh/7sgne29ut1wjlnh/D0o5K05N90You'll also need to add the codes for the character you're replacing to your RSBE01.gct.
As for the mods based on any character, right now there's .rels to port Marth, Ike and Link over almost every other character, I'm not sure how to make .rels though.