Letters should be folded into three equal parts to be able to fit into a standard-sized envelope. The fold can occur in multiple ways, according to EnglishPlus.
There are a number of different ways to fold a standard 8.5 by 11 inch piece of paper so that it fits into a standard-sized envelope. So that the reader opens the letter and sees the front-side of the paper first, it is important to fold the paper in a way that does that. The first way to fold a letter is to lie down the letter onto a table or other flat surface with the top of the letter at the top, seeing the letter how one would read it.
Take the bottom half of the paper, fold it up one-third of the way, then take that fold and fold it again to the top. This way, the reader will have to unfold twice to see the letter from top to bottom. Another option is to fold over. In this strategy, it is easiest to fold the bottom half up one-third of the way, then to fold the top third over the new fold. This strategy allows the reader to see the top of the letter with the bottom hidden.