Take a look at the parts fische at:
http://www.hdlparts.com/fiche_section_detail.asp?section=148103&category=MOTORCYCLES&make=HONDA&year=1998&fveh=3466Or one of the other parts suppliers on the web.
The way I've done it is to slide the O-ring down the tube of the petcock and then the screen; install the petcock assembly into the tank. Leave the nut a bit loose so you can adjust the petcock to align with the on/off/reserve lever.