How do you write a letter with postscript?

'Postscript' can either be a programming language or an addendum to a letter.

If you mean an addendum to a letter, then you would not plan to write a letter with a postscript in advance you would include what you wanted to say in the body of the letter. A postscript is added after you have finished the letter (signed it) but realised that there was some extra information you wanted to include.

If you mean Postscript the programming language, then you would need to go on a programming course to learn how to program in Postscript before you could write a letter using it.