Like so, although personally I would have the buckle closer to the pedal. A pull upward on the strap tightens it. Thumb outward on the top of the buckle loosens it. Some people tuck the end through the buckle and don't tighten the strap at all, but if you're using it as intended, leaving the end free is the way to go.

Other than that, the bike looks amazing. Super job!