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 That's all there is to it.

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