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
Step 2: First Fold
Step 3: Second Fold
Step 4: Loop Fold Up
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...
Step 6: Tuck and Go!
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.