The tri fold is the easiest way to fold prefold diapers.  However, it does come with the most variations.  The tri-fold with fold down technique is illustrated with a Bummis Super Whisper Wrap in a Froggy Print.  Note two of the distinguishing features of the Bummis Super Whisper Wrap--the front pocket to hold the prefold diaper and the overlapping velcro tabs for a smaller fit.

Simple Tri-fold:  Fold the side panels over center thicker panel and place inside the diaper cover.  

Short tri-fold:  Fold the diaper into thirds horizontally bringing the stitched edges of the diapers to lay over the center.  

This puts all the thick portion of the diaper in the center.  This fold is nice to use for a girl or if your diapers are a little longer than the diaper covers when using the Simple Tri-fold method.  This fold works better with the standard prefold diapers.  You may find the center section of the premium prefold diapers to be too bulky for this fold.

Tri-fold with fold down:  This is another option to use when the prefold diaper is longer than the diaper cover.  Fold down one stitched edge of the prefold diaper to make the diaper the preferred length.  Then follow the Simple tri-fold instructions.  

Place the bulkier section in the front portion of the diaper cover for a boy or the back section of the diaper cover for a girl. (Many people do not like the extra bulk of the diaper placed in the back.) The Short Tri-fold option is actually a better fold for a girl.

Fold down front or back of prefold diaper until desired length.  (front is preferred for boy)	Fold left panel of prefold diaper onto the center panel.
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Fold right panel of prefold diaper onto the center panel.	Place folded diaper into diaper cover.
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Close cover securing Velcro so the cover fits snug around the waist and leg openings.	(Check to make sure all diaper is within the cover) Success!