Firstly, welcome to SimV 
When you download a skin, more often than not there is a Readme file that will give you instructions to follow. If you find a file that does not have a Readme, first try and load said skins by following the Readme of another skin. This has worked for me often (I am not the most computer literate person you will ever meet ). If that does not work, you can always post a question about a particular skin you are having problems with. Here at SimV we have a number of people that can work minor miracles when it come to that sort of thing.

Matt