A copy of “How to Write Mathematics,” by Paul Halmos, originally published in 1970 in L'Enseignement Mathèmatique is available as

• a PostScript or PDF file

suitable for viewing,

• a Postscript file suitable for printing

on a duplex Postscript printer in booket form. (Print and fold.)

The essay is one of the classics on the art of expository writing — has lots of good advice, and is fun to read.