This instructable is for 1:24k scale 7.5 minute quadrangles in the U.S. For other countries and maps, you may need to experiment a little.
Step 1: Align the First Fold
The first fold is the most important. If the first fold is not aligned, you'll get misaligned north south creases. Lay the map face up and fold along the vertical axis. You should then have half a map and the white side should be visible. Line up the edges well and crease.