How to install:
Download (or extract) the .bsp file, browse to your C:\Program Files\Steam\steamapps\[SteamID]\[Game]\[Abbreviation]\maps folder and insert it there. Now simply run the game and play! (an example of my location for Half-Life 2: Deathmatch levels is C:\Program Files\Steam\steamapps\saldrfingerfood\half-life 2 deathmatch\hl2mp\maps)
Common Game Abbreviations:
Many maps have two or three letters before the name of the map that indicate which game it was designed for or works best in. Common abbreviations are:
arena, cp, ctf, pl, and tc = Team Fortress 2
cs and de = Counter-Strike
d1, d2, and d3 = Half-Life 2
dm = HL2: Deathmatch
(?)dod = Day of Defeat
ep1 = HL2: Episode 1
ep2 = HL2: Episode 2
gm = Garry's Mod
l4d = Left 4 Dead
zpo = Zombie Panic! Source
zps = Zombie Panic! Source
l4d_assault