Step 5: Create a closure
Keeping the wallet closed can be as simple as a loose rubber band or binder clip, or as complicated as you want. What you use will heavily depend on your cover material. Paper can't take much (e.g. rubber band, tabs), fabric can take a great deal more (e.g. buttons with string, snaps).
For my paper samples, I chose to make a tab for one and a captured rubber band for the other.
Captured Rubber Band: Find a light-strength rubber band that is barely snug around the width of the closed wallet. Cut the cover-flap from the end into thirds width-wise---cut maybe 1/3 of the length of the cover-flap (I cut up to 1/2, but I think 1/3 would have been better). Fold middle tab in half, glue it to itself. Fold again in half, place rubber band in the fold and glue down, capturing the rubber band. Fold side tabs and glue to give it a little extra strength.
Tab: Cut a slit into the middle of the "front", the side covered by the top cover-flap. In the cover-flap, cut a tab as wide as the slit and all the way up to the level of the slit underneath. Fold over and glue as you wish---I made a double-thickness tab with the sides folded over for some added strength.