In order to use SkinShifter, you first need to install the GuiLib library. Instructions are given on the GuiLib Setup page. Note that GuiLib only needs to be installed on the client.
Afterwards, you can continue with downloading the actual SkinShifter mod from the official download page. Make sure that you download the
main JAR and not the
dev one! Then, just drop the downloaded JAR into your
mods/ folder, restart Minecraft, and you're ready to go!