1. look for references of things that you think could be related to your future spawn. Nature is very much your ally here, as everything is sort of creepy. But honest, I think even regular household items can be sent to fucked up land. Feelings and abstract thingies also do wonders as extra ingredients.
2. Conneeect with your inner creepness, whatever it may be. We all know it's there. Wait till you are properly inner-ly unsettled. (this state of mind applies to drawing almost anything, btw, not just monsters)
You are done ^^