Very simple really -
Cut a large square out of a piece of paper - and fold in half to make a triangle shape.
Place triangle with right-angle at top, the fold at the bottom, on table in front of you. Take right hand corner and fold across to left hand side so that it meets the left edge just over half way up.
Take left hand corner and fold over to the right hand side so the point meets where the right flap has folded from.
Fold the top part down, and tuck into the left hand flap, this will hold the envelope closed.
Hope this makes sense! ;D
