Installing custom content
To install and play a standalone Quake map, it needs to be placed in the default maps folder. For most maps on Quaddiced, this is under \quake\id1\maps\. If the directory does not exist, for example after a fresh installation of the game, it has to be created manually.
All maps on Quaddicted are hosted as ZIP archives which can be opened and extracted by various tools such as 7zip. They usually contain one or more files with a BSP extension - these are the actual Quake maps - as well as a text file with information on the level and its author. The easiest way to install it is to extract the contents of the ZIP file directly to the maps folder. If it is extracted to a different location, all files have to be moved to the maps folder afterwards.
Once a map has been installed like this, it can be loaded in the game by bringing down the console with the ~ or ^ key, and typing "map mapname" (without the quotes) where mapname is the name of the (first) BSP file. This will start the map on Normal skill. The difficulty can be changed by using the "skill #" command in the console before loading the map (# is the skill number from 0 through 3, easy through nightmare).
There are also some maps with additional, non-standard features that require a mod ("modification") to work correctly. This is mentioned on the description page of the level and in the text files that accompany the release. In this case, the map has to be installed in the maps folder of the particular mod, for example \quake\quoth\maps\.
Some maps or episodes come with additional content and game features that go beyond the standard Quake code. These are refered to as mods ("modifications") and can be identified by the presence of a progs.dat file or one or several files with a PAK extension. Unlike normal maps, mods need to be installed in an individual folder in Quake's root directory, for example \quake\quoth\. If the ZIP already has a mod folder, it can be extracted or copied into the Quake folder; if it does not, the mod folder has to be created manually and the contents of the ZIP extracted into that. Sometimes it is necessary to manually create a mod folder as well as maps folder inside it, and copy the files from the ZIP accordingly.
A correctly installed mod either looks like this: \quake\modname\pak0.pak or, if it does not come in a PAK, like this: \quake\modname\progs.dat & \quake\modname\maps\modmap.bsp.
To start the mod and load the map, the *-game modname* parameter has to be added to the command line of the game, for example glquake.exe -game quoth. Once game and mod are running, the map can be loaded as described above.
Installing graphic modifications
Graphic modifications such as high resolution texture replacements need to be placed in the \quake\id1\gfx\ folder. They will be loaded automatically when starting the game, provided the engine port used supports such material.