This article is from the GNU Chess and XBoard FAQ, by Tim Mann email@example.com with numerous contributions by others.
First, you need to get Crafty and install it as a command-line
application on your machine. See topic [F.3] .
To use Crafty with XBoard, give the -fcp parameter like this:
xboard -fcp "crafty" -fd crafty_directory
Here crafty_directory is the directory where you installed Crafty. You
can add more xboard options at the end of the command line.
Crafty 15.14 or later is required to work properly with XBoard 4.0.0 or
later. We generally recommend using the latest versions of both XBoard