Manually = put the folder in C:\Users\Admin\Documents\Rainmeter\Skins (if you're using windows7). Or, if there's a bunch of files/folders in the zip - make a new folder, name it and unzip it to the same place (this is for mostly for older skins - but sometimes people don't package it up in .rmskin format).
I would suggest that you go into Windows Explorer and look at the Rainmeter folders that are in C:\Program Files\Rainmeter and get familiar with what's there. Just remember that the 'skins' are filed in C:\Users\Admin\Documents\Rainmeter\Skins and everything else is in C:\Program Files\Rainmeter.
For XP, if I remember correctly, it would be For XP C:\Documents and Settings\All Users\Documents\Rainmeter\Skins.
And remember to REFRESH ALL after adding a new skin if it wasn't a .rmskin install!!! And REFRESH SKIN after editing a skin!!!