How do I get my homemade Marshmallow fondant to not sag or bubble? What am I doing wrong?

What size recipe are you using? What is your method? Are you kneading it long enough? Are you using it right after making or do you let it stand at room temp for an hour or three?

The only thing I can think of is that its perhaps too soft, but I've only ever made two batches of MMF and the first one was (kind of) a fail haha.

I always put mine in the fridge for a couple of hours - I put a coating of crisco on it and wrap it tightly in plastic wrap. If it's still a little saggy after kneading it, you may need more powdered sugar.