...She supposes it would be possible to spell something so that it would catch fire later in that way. And throw it, if for some reason that came up. Maybe historical army mages did that at some point. 

She looks to Yuna for indication of what she's supposed to be doing here. 

Yuna is watching Andrew, applauding and laughing every time one of his projectiles hits. 

She'll stand where she is and watch then.

“Don’t want to join in, Elvira?” Andrew calls.

-They don't want her kneeling and pleading when they say things like that- (hopefully that part remains when it's Andrew-like-this (if she does understand correctly she's no longer sure field cover is currently the correct term) saying it.)

"I'm sorry - am I intended to?"

"Only if you would like to. If you want to practice some magic."

Well, they presumably want Andrew doing it, which is her best source of guesses at the moment aside from Yuna's own actions. (This does not at all answer the question of what work she might be doing, but. Such it is).

"Thank you."

She walks to the yet-unspelled projectiles, waits for the permission from the collar controller that will let her use magic. 

Yuna gives that permission, and very happily.

She's not sure if she's supposed to be doing the fireballs in particular or projectiles in general. Learning the fireball structure sounds interesting if she may.

"May I examine one of them for the spell structure?" 

"Of course! Have at."

She looks to Yuna again to be sure this does in fact mean she is allowed to do so.

Yuna nods, smiling. 

She starts examining a fireball projectile.

It’s a simple enough ball shape, spelled to burst into flame on contact. 

She starts examining the spell structure.

This (looking and it-looks-like-she'll-be-copying) is not, of course, how one generally learns new magic when one is training. But she's done it in the field, and as training for the field. (Of course that version comes with the don't-get-ideas after, when one is back to base.)

"Need a hand getting started?" Andrew offers.

She doesn't know what he means by that, and indicates that.

Andrew demonstrates how to activate and throw them.

She pays attention. 

"May I try working on one?"

"Please!" Andrew gestures to them.

Requisite look to Yuna

Another encouraging nod.

The rules are more than habitual, carefulness of the long-automatic sort is more than habitual. She copies exactly, she checks the one she was examining to get it right or as close to as possible. She notes it in memory, as she can, in case she's asked or going to be supposed to remember. She does not linger too long, she does not do anything even suggestive of contact with Andrew while she works. 

Andrew continues practice with the projectiles while she does so.

She works, she checks over. She watches Andrew. When she's done she steps back and waits. (She made some mistakes - obviously, this being her first time at this spell. She doesn't know yet how strict they'll be here about that. (Or if they're going to also pull out something completely bizarre again.))