So first of all go into the core files of Halo PC / Custom Edition at:
C:/program files/microsoft games
Find the folder named "Maps" and open it.
So lets just use the modded version of bloodgulch as an example. Find the file called "bloodgulch.map" and cut + paste it into another location (You might want to remember where you placed it so that you can uninstall it). Now drag the downloaded map into the folder (Maps).
Now when you go into halo, load up the map same as always and play! It's as easy as that!
Uninstallation is quite literally just the reverse, delete the modded map and drag the original back into place. Just make sure you NEVER rename the maps as it may come up with an error at the start of Halo. Unless you are using Custom Edition, then you can rename to whatever!
Hope this helps