Weird, unexpected images might flash into your brain, and not all of them are going to do it for you.
Try to stretch your desires beyond your past experiences and immediate affections, to something you’d hypothetically want if you had the opportunity. Pick out the best parts, even if they were only flashes of pleasure and excitement. How do you get those fantasies flowing? You don’t mention in your question whether or not you’ve had sexual experiences with others, or even whether you’ve had crushes, but if you have: Ruminate on those.
