To fold an envelope, start by folding the top left corner in to the top right corner. Fold in half again and then fold in half a second time. Place it in front of you in a diamond shape.
For the next step, you want to put the point of your left index finger on the bottom middle tab and fold it up to meet with your pointer finger.
Put your pointer finger on the right tab and lift up while pushing down with your middle finger against the backside of the upper tab. Keep pushing down until you reach halfway down and then release all pressure from your fingers.
The last step is to push the bottom flap up at a 90 degree angle. Flatten it down and repeat on the other side.
Now you have your perfectly folded envelope!