“I have made this letter longer than usual, because I lack the time to make it short.”
Take a first draft of something you have written, that you like, and cut it by a quarter. Then cut it by another quarter. Do four revisions, each time cutting it further. (It’s okay to add a word or sentence or phrase as you go, as long as you keep honing your overall word count downward.)
An alternative to this exercise is to choose a set number of words—say 300 or 700 or 950, and make your piece fit that space, with no extra words..