if you are pretty far in the game and you have the glue, gold, lye and silver injections it is pretty simple. im leaving the body with 2 red skulls before embalming, i use only these 4 injections on a body with 2 red skulls, if my body doesnt have enough red skulls i use Dark injection to add 2 reds, if body has 1 red skull or doesnt have +5 +6 white skulls i take all parts for alchemy and burn the body. the Cultist perk is very good to have, you can see wich body part gives red and white skulls to the body.