You can make an origami envelope by making a few quick folds of a sheet of paper.
Glimpse of the Result:
Fold a square sheet of paper in half, make a crease and unfold.
Lift the bottom part of the paper and fold it upwards as shown.
Fold the corner of this fold in a triangular shape.
Repeat the previous step for the other side of the fold.
Now fold both the sides of the paper inwards along the triangular fold.
Make a triangular fold at the top by bringing both the top edges towards the center.
Bring the top downwards and insert the top triangular part into the socket below to finish the envelope.