Depending on whether you have the auto-installer or the extraction. The first will do all the work for you - just point at the Doomsday folder CORE is in and go), the latter will require some tinkering (albeit a little). It's best to rename your current gfx folder, extract the graphics pack into the Doomsday folder, and then copy/paste the original folder back in. The reason is that we have to add some dummy files to the original file structure in order to have our graphics load succesfully. The auto-installer will verify no existing graphics will be overwritten (usually expansions/graphics mods), but otherwise this method will ensure you don't.