It is not necessary to buy envelopes. They can be made quite simply out of paper. Creating your own envelopes saves you money and allows you to add your personal touch. If you get good at making your own envelopes you can start selling them to others and make a little extra money on the side.
Choose your paper. You could use a coloured piece of paper or a plain sheet of white typing paper. If you want a larger envelope, buy legal-size paper. This type of paper is often used by solicitors and estate agents for their contracts. You can get it pretty cheap at shops like W H Smiths or Asda.
Fold your paper 1/3 of the way up, and make a crease. Run your thumb nail across the crease so it stays.
Unfold your crease, then fold in each of the two sides 1.2 cm (1/2 inch). This is going to make the edge of your envelope. The fold will give it a smooth edge and keep people from getting paper cuts on it.
Fold your bottom back up from the original crease you made. Your edges are folded still, and you will now need to glue them so they stay shut. Use the liquid glue -- not glue sticks. Wait for the glue to dry before you proceed to the next step.
Get out your scissors for the top. Now you have the bottom of your envelope finished you need to create the flap that folds over. This flap is usually a triangular shape, so cut each side in a diagonal manner until both sides meet at a point.
Use double-sided tape to get your envelope to stick. Place one strip down each side of the diagonal lines you just cut in the top flap. Make sure the strip ends just before the edge so that it doesn't stick to any letters inside of the envelope.
Place your letter inside the envelope and seal shut. Just press down along the edges to make sure everything is in place. Add your addresses and postage, and place in the postbox.
Buy some sheets of scrapbooking paper, which has all sorts of designs. This will give your envelopes some character and make them more fun to receive in the mail. You could staple your envelope's sides, but it doesn't look as nice.
Be careful that you don't get a paper cut. If you do, be sure to clean it out right away.