Installing Custom Maps
If you need any more help after reading this guide, please ask in the forum.
Most .zip and .rar files on this site are meant to be installed to your half-life folder, which is usually C:\Program Files\Steam\steamapps\ACCOUNTNAME\half-life. Everything will automatically go where it's supposed to.
Simply start Sven Co-op, go to Start New Game, and pick the map you want to play.