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It should be plain and either square or rectangular. It should not have designs on it. However, it can be stylized--you could have custom envelopes made that are thicker and more durable than normal envelopes. your letter so that it fits into your envelope. Make sure to fold your letter in one try as a letter with many creases and re-creases looks unprofessional.  If you are using a rectangular standard envelope, fold your letter into horizontal thirds. If you are using a square envelope, fold your letter in half horizontally and then in half vertically so that is forms a rectangle that can fit into the square envelope. Seal the envelope by licking along the seal line or peeling off the strips covering the sticky part of the seal (depending on what kind of envelope you have bought.) Write your name in the top left corner. Write your street address on the line below your name. Write your town, state, and zip code on the line below your street address. In the lower right third of the envelope, write the name of the person you are sending the letter to. Write the company name on the line below that (if there is a company name). Write the street address on the line below the company name. Write the town, state, and zip code on the line below that. Make sure your stamp is worth the weight of your letter.
Pick out an envelope.  Fold Place the letter into the envelope. Flip the envelope over. Address the envelope. Place a  stamp (or stamps) in the top right hand corner.