A quick, self-locking fold for an 8.5 x 11 (or A4?) piece of paper.
Resulting size: Approximately 2.5"x4.25" (if A4, 87x105 mm -- I don't have A4 paper, but the math should work like that).
Step 1: Take a Blank Letter/A4 Paper
Blank paper, Portrait
Step 2: First Fold
Fold upper right corner to left side, making a 45 degree right triangle above the rest of the paper.
Step 3: Second Fold
Mountain Fold (away, behind) the paper vertically in half, so the right half is behind the left half.
Step 4: Loop Fold Up
The next step is two folds:
Fold the bottom part up along the line created by the bottom of the triangle/trapezoid.
Fold the bottom up again, the same size as the previous fold.
Step 5: Almost There...
Fold the triangle down over the rectangle (Picture is rotated 90 degrees).
Step 6: Tuck and Go!
This one is a bit tricky -- It's an optional Z fold.
The idea is to push the triangle into the crease, and here's your choice: Either leave a bit of the triangle tab out, or tuck it all the way inside. If you leave the triangle tab out, try to keep the tip below the outer rectangle fold, and have the triangle tab lined up with the other triangle in front.