First determine what you would like your inner liner to be. Add a splash of color with paper in the shade of one of your wedding colors. Paper doilies add a touch of elegance. You could also get creative and do something a little more fun like a map. No matter what your envelope liner is the steps are the same.
Step 1: Lay the liner over the envelope (if a doily then poke out all the holes and lay it over the envelope with the center of the doily positioned just under the adhesive of the tip of the envelope with the wrong side up).
Step 2: Using a ruler and a pencil, mark up where you plan to cut. Cut along the path.
Step 3: Slide the cutout into the envelope to make sure it fits and fold it down along the crease of the envelope.
Step 4: Glue the liner inside the envelope. Apply glue to the lower half first and slide inside the envelope and make sure it is secure. Then glue and secure the top part.
That's all there is to it. Now sit back relax and wait for your guests to be impressed :)