1) Depending on the size of the envelope and the size of the letter, you will need to fold differently. This is for a standard formal A4 letter, you should fold twice, resulting in three sections.
2) Bend the letter (without folding) into an ‘S’ shape until the three sections look roughly the same size. If you have the envelope to hand, you can check that it will fit. The start of the letter should face outwards. If you are using an aperture envelope, it is especially important that the address is facing outwards and will be positioned correctly when it is inside. The sections do not have to be the same size; it is convention to have the front section slightly smaller than the others.
3) Fold down firmly, and use a ruler to press on the fold, making it neat, straight and crisp.
4) Place it into the envelope so that it is facing out and with the top of the letter at the top of the envelope. If it helps, imagine you are using an aperture envelope.
5) If you are hand-writing the address, do so before you put the letter in the envelope, otherwise you will dent the letter.