With skins you can just extract the contents of the zip/rar you downloaded, straight into your redfaction/user_maps/multi folder.
You only need the 0_ in some instances if it's a new model file. As for mods, extract them to redfaction/mods/"mod" folder.
( "mod" being a folder named the same as the mod you downloaded, eg say you downloaded something called fluffy-mod, create a folder within redfaction/mods/ named fluffy-mod and extract the zip/rar you downloaded to the new folder
As for running a mod; open up the RF launcher ( redfaction.exe) click the setup button, then the misc tab and type:
-mod fluffy-mod
Notice the space before and after the -mod bit