Installing fax4CUPS is very easy:

0. Install efax (http://www.cce.com/efax/) or HylaFAX
(http://www.hylafax.org/). For HylaFAX you need also grep and sudo.

1. Install the man page.

2. Install the efax/hylafax scripts in the CUPS backend directory, for
instance, /usr/lib/cups/backend/; you can look at the first lines of the file
for configurable parameters (the fax script is provided for backward
compatibility only).

3. Install the PPD files efax.ppd/hylafax.ppd in the CUPS model directory, for
instance, /usr/share/cups/model, and create a new printer using the a suitable
backend (see the man page; the fax.ppd PPD file is provided for backward
compatibility only).

4. Restart the CUPS daemon (usually "/etc/rc.d/init.d/cups restart" as root
will do the job).