To complete your game board, you'll need to attach the leg assembly to the frame. Turn the assembly upside down on a clean table or flat work surface, and slide the two bolts (with a flat washer on the head) into the holes in the frame so they just barely protrude through the frame. Slip another flat washer onto each bolt inside the frame, then move the leg assembly into position (as shown on page 1 of the Free Woodworking Plans). Slip the bolts through the holes in the legs, and then add another flat washer and a lock nut to each bolt. Tighten the lock nut so that the leg assembly is secure, but not so tightly that the legs can't fold under the board.
Turn the assembly over on your work surface and open the legs so that your new board appears ready for play. (Notice that the bottom of the legs won't quite be parallel with the ground, because you'll still need to attach the 1/2" deep rubber bumpers to the legs after applying the finish.)
At this point, it's time to apply your finish of choice. The only really important part of finishing a cornhole game board is the playing surface. For this surface, you'll want to use a high-gloss paint which will allow the bags to slide somewhat on the surface. Semi-gloss or flat paint simply don't allow the bags to glide well enough.
Apply a few coats of gloss paint to the playing surface of your board until you're satisfied with the playing surface. As for the finish on the rest of the assembly, that is up to you. You may choose to finish the frame and legs with the same high-gloss paint or maybe use polyurethane for protection. Just be sure to remove the leg assembly before finishing and then re-assemble after the finish is complete. Add the rubber bumpers to the bottom of the legs to complete the project.
Don't forget to make your own bean bags too!