I've never done it on Debian but have done it several times with CentOS. Basically, the box needs to have a TFTP server and a DHCP server (if there's not one on your LAN already). Then you need to get PXELINUX (which is part of the SYSLINUX package). You'll have to put the PXE Linux stub, kernel, and initrd for booting into the directory that the TFTP server serves out. Then you have to modify the DHCP server to send clients (you can base it on MAC address) to get the boot files via TFTP from your server. You do this with the next-server and filename options in dhcpd.conf.
The PXELINUX documentation goes through this in some more detail, and there is information available on the net if you google a bit, but this is the process in a nutshell.