There are two basic approaches.
One is to use the existing bench and the porta-potty recess as the footwell. This is the simplest and allows you to retain the upper bunk if desired. Basically you cut the
fiberglass around the footwell, box it in for support, cut a table to fit, and mount it on a pedestal or attach it to the front wall.
The other is to remove the bunk, fill in the recess to create a level floor and build new benches up from there. It's a lot more work, but it allows you to have a larger table, to sit higher for a better view out the front window, and to offset the table toward the door side for easier access to the galley-side seat. That how Scamp does it on the deluxe models. They also add a storage closet on the galley side against the wall, since that seat is deeper.