Unfortunatly there really isn't any type of step by step guide for setting up an Xserve since there is so many diiferent ways you can depending on your needs. I would recommend this
book. It does a good job of covering the main features of 10.5 server and shows basic setup examples. You won't be using all the options but is a good intro. It can be dry reading but does cover things in some detail. There were several good books on setting up 10.4 server but they aren't really applicable to 10.5 since the server application was changed in the newer os.
Also check out the Apple discussion boards for OSX server.(
here)You can find answers to things you might not find covered else where. I have searched there more than once to find out how to fix bugs in my setup.
My advice is trial and error. I reinstalled 10.5 server on my Xserve several times on a second drive while still running 10.4 server. It allowed me to try several install options and learn the changes and to find out which configuration worked best for my setup.(ended up using the 'advanced' option on the install config) Good luck.