Sometimes, you can use MODs designed for other skins/textures on other skins/textures.
This rarely applies on vehicles, but 99% of the time, it works on normal ped skins.
Follow the exact same steps as installing a MOD normally.
The only difference is that, before you drag and drop the .txd and/or .dff file onto Spark program, you have to rename the file first to an appropriate file name.
Check online for specific file names for certain skins
Change BOTH of the appropriate .txd and .dff file names, and then proceed with the installation of the MOD.
Remember! MOD-ing is pretty much like programming, and in IT, sometimes things don't work out the way they are.
So, enjoy MOD-ing, and if things don't work out, don't get frustrated.
If you need help, just post a question and someone will answer it for you..