In need of help

Well, my ideas usually just hit me, whether it be while listening to music or walking outside. As for writing, I usually have a good idea in my head about the characters' personalities and their interactions (these characteristics/interactions should be realistic, or else the character is lost on the audience). Additionally, I have the scenario planned in advance, and I usually avoid the typical Wonka scenario (unless absolutely necessary). This depends on what pleases you, really.

Of course, when writing an explicit piece, character development doesn't matter as much, but unrealistic dialogue is risky and is likely to turn its audience off. As for the inflation itself, write all the details that pop (no pun intended) in your head. The gurgling, the moaning, the hissing, etc. Also, write down what is happening to the inflatee(s) and/or what the inflator(s) is doing to the inflatee(s). The gender of the inflator/inflatee shouldn't matter as much beyond physiological/anatomical differences (and others, depending on the culture). 

Dirty talk can be a bonus, if necessary. 

Those are just a few tips I have in mind. Really, when I write, I just put whatever makes sense on paper and write a coherent (or lack thereof if I'm being very experimental) narrative.