True, you should be trying to take every combo to its fullest potential with Falcon, but let's slow down here. One step a a time, you know? Falcon should always be moving against Marth: Dashdancing, wavelanding, moonwalking if you know how. This movement and spacing can be used to bait out attacks that are easily punishable with a shffl'd knee, stomp, or raptor boost, or grab. Once he's in hitstun, upairs and nairs are always good choices to carry him across the stage for a knee or other finisher.
The tricky part to all this? Actually starting the combo. Be very, very weary of that sword and what it can do at any given time, and you'll be fine.