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.
Step 2: Fold the Sides Back
Step 3: First Horizontal Fold
Step 4: Finish Folding
Fold the top layers to the crease like you did in step 2. Repeat for both sides. If you did things well, you should see the map title on both sides, making it easy to grab the one you want.
Step 5: The "Waterproofing"
Put the map in the bag and seal it up. You'll notice it acts more like an inflatable pool toy than a bag. My Dad came up with a genius way to solve the pool toy problem.
Put the edge of the ziplock in between your canine teeth, bite down, and pull out. If you're feeling really adventurous, get a pair of scissors and cut a small slit in one side of the bag. The hole is small enough that air gets out, but unless I really use it as a pool toy, water doesn't get in. I've been doing it this way for years, and I haven't ruined a single map in inclement weather. If you're worried about the waterproofing, don't cut the hole.