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.  

Simple Tri-fold:  Fold the side panels over the center, thicker panel and place inside the diaper cover. For this method the prefold would be a comparable length with the diaper cover. 

Short Tri-fold:  Fold the diaper into thirds horizontally, bringing the stitched edges of the diapers to lay over the center.  For this method the lenght of the prefold is generally longer than the diaper cover in use.

This fold is preferred for a girl when your diapers are a little longer than the diaper covers when using the Simple Tri-fold method.  This puts all the thick portion of the diaper in the center.   This fold works better with the Standard Infant or Baby Prefold diapers.  Most people find the center section of the Premium Infant or Baby Prefold diapers to be too bulky for this fold.

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

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. The Short Tri-fold option is actually another suitable 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.  I do need to update this picture. It is better to NOT put the prefold inside the "pocket" in front of the cover.
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Close cover securing hook and loop so the cover fits snug around the waist and leg openings.	(Check around legs and back waist to make sure all of the diaper is enclosed inside the cover) Success!