Step 1: Choose paper the same weight as copier paper or lighter. You'll need a spare envelope for a template.
Step 2: Unfold spare envelope. Using a ruler and pencil, draw lines about 1/2 inch from edges of flap (just inside glue strip), sides, and about 1/16 inch above bottom fold.
Step 3: Lay template on stack of paper (you can cut through up to 5 sheets at once); cut around it.
Step 4: Slip liner into envelope and center.
Step 5: Holding liner and envelope in place, fold flap over and crease.
Step 6: Lift envelope flap, and apply glue to back of liner flap with glue stick.
Step 7: Fold envelope flap back down, smoothing to adhere to liner.