This is a Conway buckle

Insert one end of the strap into the buckle

Press the hole onto the buckle prong

Pull the strap through the other end of the buckle

Take the other end of the strap in circle towards the buckle

Pull the strap into the buckle, on top of the first strap

Press the hole onto the prong, on top of the first strap

Pull through the other side of the buckle

This is what it looks like when you're done, two straps overlap through the buckle

Side view of what it looks like when done. You have secured a leather strap in a round using a Conway buckle!