The basic idea is to use the Z style file zed.sty, available
below. In your document, the first line should be something
like
\documentstyle[12pt,zed]{article}
(note the `zed' inside the square brackets corresponds to the
file zed.sty which should be in the same directory
when you compile the LaTeX file).
Download
zed.sty
The new commands that the zed.sty file allows you to use
to write schema are explained in the manual: