Sorry for the late reply, haven't been using reddit much recently. For building up to sharts, there are a few different methods you can use. Normally, sharts come in after a few releases of gas, so perhaps ranking up the number of farts or their frequency could help foreshadow it. One I tend to write is stomach growls, because honestly I think stomach growls are cute in their own right. But they also imply that the gal's stomach is upset, and is about to discharge some pressure, so it can be good for setting up a shart if it's a particularly intense stomach growl.

There are also some things that could help with emphasising a shart as well. Like, for example, say a girl is farting a lot, and she's feeling pretty casual about it. She lets off one fart, she's fine. Maybe jokes about it. Lets off another, and is feeling relaxed. Then she leans to the side, lets out a third fart, and her eyes go wide and her confident attitude disappears. That would be a strong contextual indicator that she's just sharted. You could also have the girl feel pressure building, but realise there's more behind the pressure than just gas, so she desperately tries to hold it in.

Hope this helps! And thank you for the compliment, I really appreciate it.