Get a paper that's about twice as large as your desired envelope size. If in doubt, a standard size of 8.5 by 11 inches will do fine; you can fold and cut it in half before beginning if you want a smaller envelope.

Fold the paper over evenly. You should get a rectangle half the size of your original piece.

Tape together the open left and right sides. Use tape to securely close the two open edges of the rectangle, leaving the top open. The top is where you will insert your letter.

Fold down the top to make a flap. Make a small flap by folding down the open edge of the rectangle. This will keep the letter from falling out of the envelope. A flap that is about 1/2 of an inch will work.

Insert the letter or card. Bend back the flap, and insert your letter, card, or other contents. Fold the flap down again after this is done.

Glue the flap to keep your message enclosed. Place a thin line of glue along the inner edge of the flap, then press the flap down. This will keep the envelope closed until the recipient opens it. You can also secure the flap with decorative tape or a sticker.