P.R. Halmos:

    There is no recipe and what it is.

The basic problem in writing mathematics is the same as in writing biology, writing a novel, or writing directions for assembling a harpsichord: the problem is to communicate an idea.

To do so, and to do it clearly, you must

• have something to say,
  
• have someone to say it to,
  
• organize what you want to say,
  
• arrange it in the order you it said in,
  
• write it, rewrite it, and re-rewrite it several times,
  
• be willing to think hard about and
  
• work hard on mechanical details such as diction, notation, and punctuation.
  

That's all there is to it.

See here for the full text by P.R. Halmos as scanned off the Web.