Well, here is the process that I would recommend:
- Come up with a phrase or commonly used word.
- PICTURES. Pictures are an easy way to represent a phrase/word. Another way to come up with the definition part of the cryptic is to use a very vague definition/synonym/whatever that correlates with the definition and incorporate that into the rebus in some way that gives a hint towards the final answer.
- Just do whatever for the wordplay part! Using prepositions are easy if present in the phrase or in a word in the phrase (e.g. the preposition "on" in "bony"). For a clue like "bony", you could incorporate a B on top of a Y to make B+ON+Y. For the definition, just add like a few bones on the sides. This might not be the best example, however, as it should really be more creative and harder than that.