It's very simple, in such mods you get a .wtd and .wdr file, open up SparkIV. Select 'browse' then navigate to pc/models/cdimages.
In that folder you'll have a few cdimages, amongst them there's 'vehicles.img' which is for vehicles as you might've guessed, weapons.img for the obvious and playerped.rpf for player clothes. componentpeds.img is for pedesterian models.
Now let's say you want to install a weapon, for instance the regular pistol. Just open up the weapons.img and select the import button. Now select the .wdr and .wtd file that came with the mod and import those. The files you replaced should now be highlighted in blue, hit 'save' and you're done. Cars and clothes are done in the same way.