If using a T - shaped fastener (like a Snappi), hook the two top ends of the fastener to the left and right flaps, then hook the bottom end to the bottom center of the diaper's front. (babycenter.com)
My one quibble is that the thick center section of the prefold is not always in the center of these diapers, which makes fastening them with a snappi bulkier on one side than the other. (tinytush.com)
Some cloth diapers come with a special little pocket slip located in the center of the diaper. (earlymomentsmatter.org)