Here are the tools you need to buy, beg, borrow or steal (JK)... A random orbital sander and a jig saw. Start by cutting a piece of cardboard with a razor knife as a template for the raft floor. make sure said template fits in the boat well. Trace this template onto the plywood and cut with a jig saw. Sand edges and face to your hearts desire.
Look for these style FLOOR FLANGES.
Attach them to the TOP of the deck at the corners and mount them.
Then put the floor in your boat and rig your frame on to the boat. Mark locations that are appropriate for strap holes so floor can be attached to the frame. Take floor out and cut holes using multiple drill holes. Remove Flanges and coat in varithane and epoxy.