Prompto stood by, giving Sarah time to hold the gun as she thought was correct. She was rather off, however, and he secretly thanked Ignis for suggesting they didn't set out right away. If they had, Prompto wouldn't have known just how much help Sarah needed.

"Well...not [i]quite.[/i]" Prompto admitted with a chuckle. "You'll want to hold the handgrip with your trigger hand," he showed her where to place her palm, careful to move slowly so she'd have a chance to practice herself. 
"For this type of rifle, keep the end of it balanced in the pocket of your shoulder, and you'll want to rest your cheek rest against here," he helped her guide the weapon to a firing position. "There! It's kind of complicated and might feel awkward the first few tries, but after a bit of practice, you'll be a pro in no time." 

After double checking that all was in order, Prompto stepped away to give her some space.
"Oh, and one more thing...remember to breathe," he added, sensing that Sarah may have been a little nervous still. "If you hold your breath, you'll get tense. Trust me, it makes aiming a lot more difficult." This was said from experience. 

"When you feel ready, you can take aim...try for the middle can there." Prompto nodded to the largest of the cans. It stood a little taller than the rest, and he figured it would be best to start off easy. "Don't worry, you'll do great!"