The minimum requirments are obviously a development station and a target. However, the recommended way of working is having a third host which acts as a server. The server runs several services such as ftp, telnet, NFS, tftp (if needed) and CVS. The main role of the server is to run CVS and track version control, however once you can boot the target from network, the server will also hold the target images, and filesystem, which makes development much easier.
Regardless, the first step is to install a tool-chain (compiler, linker etc.) for your target. The HardHat Linux cdrom includes all the needed files, and the installation sequence is documented in the HardHat Linux documentation. During the installation, you must select your LSP (basic software for the selected board), and HardHat will install a set of tools and a kernel source tree matching your LSP.
We had a board that had vxWorks running on it, so we setup the target to boot using the standard vxWorks loader. Once the loader initiated, we used visionICE to take-over the target (so that vxWorks won't load an image file) and load a Linux image into the target. What you need to do at this point is get an ICE, connect it to the network and to the target - through a JTAG connection - and install the ICE software on your host.
What should have been done so far:
A Linux host installed, and the HardHat tool-chain.
A working target (hardware should be functional)
ICE is connected to the target and the network and its software usable.
Optionally, a server running CVS, telnetd, NFS and FTP.