You have to download DarkPlaces before you can even do anything - and I'll show you where you can get the latest build from the developer of the engine itself; http://icculus.org/twilight/darkplaces/download.html - download the latest stable version there, it's a small download so you won't be waiting for very long.

Once the download is done you will have to unrar the DarkPlaces file to Quake's steam base folder which is most likely to be; program files (x86)/Steam/steamapps/common/Quake - the Quake folder is a clusterf*** of different files but don't worry, nothing will be overwritten at all.

Firstly find WinQuake.exe, create a backup folder in the main Quake directory and place WinQuake in there. Now you want to rename DarkPlaces.exe to WinQuake.exe as this is the file that Steam uses to run normal Quake.

And at this point you should be done! Open up Quake and select the main Quake option (Not GL, QuakeWorld or GLQuakeWorld - just normal Quake) and it will be running Quake on the DarkPlaces engine and the Steam overlay will be available for you to use and it will track your time with the game!

DarkPlaces doesn't come with the insanely high resolution textures, models or anything like that - you have to download them yourself as DarkPlaces on it's own is still Quake except shinier and less of a pain in the ****ing *ss to deal with. Explore mod sites and Quake sites for mods such as Quake HD and Quake Reforged for a true modernized HD Quake experience.

DarkPlaces also has native mod support - while the mod section will look like a mess due to the many Quake files inside the directory, any mods you install in that directory will be available for you to use in that section.

So have a play around, but be careful not to destroy the Quake folder - if things go wrong, uninstall Quake, delete the directory and reinstall everything again and you can start again from there.

Have fun!