To layer a spell, instead of a one shot for the outcome, you enchant for all the different parts that lead to it.
For example, in a layered love spell, instead of summoning just Sallos to bring the girl/man/anteater to you, you would call Sallos to work on her heart, Dantalion to work on her mind, Sitri to increase your own attractiveness, King Paimon to influence her friends (a woman is more likely to become interested in someone if her friends think they are good), Belial to work on any limiting beliefs or inhibitions of the target, etc.
You’re not doing it all at once though. You would task each of the demons individually, all leading to the final outcome of being with the girl/boy/anteater of your dreams.
There is a great example on the forum if you search for “layered love spell.”