I've included hard covers, and a pretty, covered band, but they're not mandatory.
On to the show!
Step 1: Gather Supplies!!!
Sheet of paper (mine was 18"x24")
Pretty paper to cover with (wrapping paper? magazine scraps?)
Glue (I like Aleene's Tacky Glue for this)
Step 2: Begin folding the paper to make the creases.
Fold the paper in half, matching the corners as carefully as you can. It's easiest if you make this fold along the long edge of the paper; for example, I folded the 24" edge so that the resulting size of the paper was 18"x12".
Step 3: Continue Folding
Next, fold one free edge over to meet the creased edge. Unfold the fold you just made, turn the paper over and repeat that fold on the other side.
You'll have a flat piece that's got creases down its length. Fold this in half to make a crease going perpendicularly to the previous folds.
Sharpen all the creases and folds by pressing them and folding them in both directions.
Unfold the whole paper and lay it flat.