I wanted to do this as I’m a speed play user and didn’t find much online so here’s my how too Speedplay + Straps.
OK you ready for this… bolt, washer, done. Almost, couple of things to note:
1 > Since were putting a bolt through the strap you only get one go at it, so, I found the thing to do is to take the pedals off, clip your shoe in and dummy fit the strap, mark where the strap over laps the pedals bolt hole and drill a small hole through the leather. SMALL HOLE and the screw the bolt through the leather. Small washer for obvious reasons.
2 > The silver “bowties” are removable and have a countersunk hole and a threaded hole so depending weather you want the strap at the front or rear of the pedal you may have to flip the “bowties” NOW you can’t use the original counter sunk bolt as once you add the strap width the bolt is no longer long enough so you have to buy one from the local hardware store. The chances of getting a bolt exactly the right length is slim to none so if you don’t own a hack saw and a file - get them while your there too.
3 > Put the bolt through the counter sunk side and tighten. At this stage if the bolt is hanging out the other side mark it, remove and trim.
4> Should now be looking like this, done, easy 
5 > Here all clipped in
Happy days