Use a standard letter-size sheet of paper
Fold the paper in half lengthwise, then unfold
Fold the top two corners down to the center line
Fold the new top edges down to the center line again
Fold the plane in half along the center line
Fold the wings down so each wing is level and even
Make the wings slightly smaller than usual
Bend the back edges of both wings upward a little
Add a small upward curve to the wing tips
Keep the nose slightly rounded, not too sharp
Make both wings and folds symmetrical
Throw it with a fast, smooth, level motion
Aim slightly upward
Use a gentle wrist flick for more looping
Adjust the wing bends if it flies too straight or stalls
Add a tiny bit of weight to the nose if needed
Test and fine-tune the wing angles until it loops