Summarize the following:
Perform a book fold to begin. The best type of paper to use is an ordinary 8 in (21.5 cm) by 11 in (28 cm) sheet of printer, or A4 paper. Smooth out the crease after you've folded the paper to make it stronger, and make the folds easier to align. Unfold the paper after you crease it so the you now have a piece of paper with a vertical valley crease.

Summary:
Fold a piece of paper in half lengthwise.