Leo,
Like blueslfb said go slow and stay away from the glass!. What I do when working risers is put two layers of quality masking tape on the face and edges of the glass. When I nip in to them with a rasp I replace them immediately. Also if ya do nip the glass use thin cyanoacrylate (super glue) to mend the edge.
Before you begin I would strongly suggest that you develop a plan for what you are going to do. If you have a grip that you like look at it and see what you can do to bring the martin in to a similar or identical shape. Mark the grip out with a pencil and slowly rasp material and then try your hand on the riser. It is amazing how little material it takes to make a major change in how the grip feels. What I have done is colored the area I wanted to remove with pencil, remove it all, try the fit, mark what I wanted gone and just repeat the process.
As far as refinishing I would refinish the entire bow when I was done. I like to use brush on minwax helmsman first, When I get the finish built up where I want it I apply the last couple of really thin coats with a spray gun.
Here is a couple of shots of a riser being completed. You are doing the same thing just starting a little further along in the process.
when you are close to the glass on the edge always make sure that you file from the glass to the wood.
After you have the riser where you want it start with sandpaper and bring it in from 100 grit to 400.I hope this was helpful.
pete