[header] How to fold a plastic bag [title] Flatten the bag. [step] Make sure to line up the sides of the bag so that the handles are parallel to each other. Pay attention to the way you orient the handles, as they play an important role in the folded triangle's structure. 
[substeps] This is easiest to do on a hard, flat surface like your kitchen counter. If you're especially dexterous, you can try folding a triangle in the air without a flat surface.