by T¹ » Apr 23, 2002 @ 4:16am
Put the .bsp file of the custom map you wish to play in \quake\id1\maps\ - you might have to make the 'maps' directory yourself.
Make sure you're using the registered version of Quake (i.e you have the registered PAK files installed), as custom maps & mods don't work with the shareware version.
The registered-version checking code can be removed from the PQ source (as allowed by Carmack in his email to Dan East) to allow the use of custom maps with the shareware version, but you'd have to edit & compile the changes yourself - not exactly an easy task for a novice.