I can help out as someone who’s done customs for a long time!
My process is a little odd, considering that I can work off of mostly anything, but usually when someone commissions a custom they will have an idea of what they want ^^
To help them communicate this idea, you can ask them the following:
-What species? Any specific colors or color palettes? What about clothes or clothing styles? Any specific accessories?
Though, some people may opt to send you a mood board or several images related to what they want instead! You can actually specify what method you’re most comfortable with (description, mood boards, color palettes) when you set up the commissions page, so people know what information to give you when you discuss the details of the commission.
Customs are usually more expensive than regular commissions, considering you have to make a whole new design from scratch. Pricing them based off the price of your fullbodies and then some is common.
Finally, I would suggest doing a couple test customs to see what works best for your creative process! You can ask your friends, or even make a thread offering customs at a discounted price (to bring in more people) and see which methods that come up work best for you!