Paper shape is the key to cooking en papillote.
How to make a parchment packet
The key to cooking en papillote starts with the shape of the paper. Fold a piece of parchment paper, about 2½ times the size of what you'll be cooking, in half and cut it into a half-heart shape. Unfold, fill one half with the ingredients, and fold the other half over to enclose. Starting at the top of the heart in the center of the dip, fold about ¼ in. of the edge toward the center and start rolling this "hem" around the edge, crimping the roll as you seal it. When the roll reaches the bottom tip of the heart, twist to seal. When using aluminum foil, simply fold foil over contents and crimp edges to seal.