See the evilwm home page for downloads. Once you have a copy of the source code, either the git repository or by downloading a tar archive, building is very straightforward.
In addition to the standard C compiler tools, you will need X11 development files installed. Under Debian, the x11proto-dev, libx11-dev and libxrandr-dev packages should suffice.
The source distribution does contain a configure script, but this is not from the GNU build system. It is a minimal bash script provided to simplify multiarch cross-builds under Debian. Instead, edit the Makefile to modify build flags for your platform.
Then, building and installing is very simple:
$ make $ make install
That's it! But if you need to install to a different prefix, you can override with something like:
$ make install prefix=/usr/local
The install process puts a file called evilwm.desktop into /usr/share/applications, so depending on your desktop manager, you may simply be able to pick evilwm from a menu. Otherwise, most managers will run Xsession, which will look for a file in your home directory called .xsession and run it (so be sure it has execute permission). Here's a simple example .xsession file:
#!/bin/sh test -f $HOME/.Xresources && xrdb -merge $HOME/.Xresources xsetroot -solid \#400040 -cursor_name left_ptr ( /usr/bin/evilwm -snap 10 >/dev/null 2>&1 & ) exec xclock -digital -padding 2 -g -0+0
The cursor shape and background colour are set with standard X tools
(evilwm won't do this for you). xclock becomes the "magic
process" (session terminates when it is killed). evilwm itself is
started in a subshell otherwise xclock would become its parent, and
killing evilwm to restart it may result in a zombie process. You could
also avoid this by not exec-ing the "magic process"; the shell
will handle child signals properly.
See here for a more complete example of a .xsession file. Obviously, it depends on a lot of external packages.
You used to be able to configure the GNOME session manager to use evilwm as its window manager by running gconf-editor and setting the key /desktop/gnome/session/required_components/windowmanager to evilwm. The package information for gconf-editor suggests this approach is out of date, however.
If you typically start X by typing startx from the console, you might need the .xsession file to be called .xinitrc. A symlink should suffice.
After starting, you should be able to bring up an xterm with Control+Alt+Enter. For more information on configuring and using evilwm, see the manual.