Once downloaded, extract the file, and drop directly into My Documents/EA Games/The Sims 2/Neighborhoods. 
Beside the name of each neighborhood, you will find the neighborhood number it has been given. 
If you already have a neighborhood the same number as the one you want, simply rename the files before 
installing to the next number for your game (ex: N050 to N130, etc) or you risk overwriting the previous file.

If you download a template instead of the entire neighborhood, simply extract the files to your 
EA Games/The Sims 2/SC4 Terrains file.