Pretty new to the character myself, so someone with more experience can correct me, but the spell side is getting the gun out or use an ability that gets it out, and select the corresponding slots to the first spell layer you want, for example, Air bullet, you select Opponent, Oscillation, Single Target, and aim the circle where you want to start from. Then lets say you want
PerfectMissiles= Type: Oscilation/Movement, Target: Opponent, Area: SingleTarget, you first place the Air bullet setup, but you then follow up by casting Opponent, Movement, Single Target ontop of the existing spell, which then gives perfect missile.

That is my understanding anyway, so id appreciate being corrected if this is misinformation.

There are multipul combinations when you fire the gun
if you find them different things happen

Just look in the description