In order to make your own envelopes, you will need a piece of paper, glue, scissors, and optional decorating tools/stickers.
Place your card in the center of a piece of paper. Fold all 4 sides in, in order to make creases. Then, fold the top and bottom down. Then, cut angles on the sides of the 2 corners. Then, cut away the edges on the top and bottom flaps. Fold the bottom up. Then, glue the sides in, with your envelope glue. You can also decorate the envelope to match your card. You can add detail as well.
Wallpaper can be used for envelopes as well. If your store-bought envelope is too big, you can remove the excess, and use the same gluing techniques as with your homemade envelopes.