Basically whats already been said. I also find that Primers don't need to be heavily coated on th your parts. All it is really doing is giving the paint something to adhere to thats not the really smooth plastic. Typically with MR Surfacer you'll want to still be able to see the colour of the plastic shine through, so a red bit would appear slightly pink. Other than that practice etc.
May be consider getting some old plastic toys to learn on. A car boot sale, jumble, or garage sale is a good place to find cheap expendable stuff.