I made this frame to display my London 2012 minifigs. It turned out quite nicely and was pretty easy to make.
First off, cut the baseplate to the required size. I made this a while ago so I can't quite remember, but I think I cut three lines of studs from one horizontal and one vertical side. Score it with a Stanley knife and ruler, and it should snap off with a bit of bending.
You can mount the artwork in the Ribba frame at the back or the front, so mount the cut baseplate at the back with the minifigs on there. You can just insert the minifig bases in the gaps between the studs, because the gap between the studs is equal to the height of a flat Lego piece. I thought I'd have to buy some mounting pieces but it's okay without.