how do you become a red jenny

This was how I did it.

You have to be good enough friends with Sera in the main game to get her cookies scene.

Then in Trespasser, you, one of her really good friends, gets the offer.

If there is moreto it than that, I do not know. But worked for me.

DOING the Pranks isn't necessary.

ALLOWING her to do them also works (I told her that I wasn't interested, but that she was permitted to do them herself).

You need Sera's friendship approval pretty high to get the option. I've only ever got it once, and in that playthrough I got her cookies on the roof scene, never had it before. Then in Trespasser you get a cutscene with Sera and Charade, I think, Hawke's cousin, where they will ask you to become one of their 'friends'. I haven't romanced Sera so I don't know if you can also get that scene or maybe it changes because you are in a romance with her. If you accept their offer you will get a slide at the end with a crossbow prosthetic/arm.

Complete all of Sera's war table missions, stay on her good side, and keep in contact with her.
I think I've heard that the end of her Red Jenny mission gives you the choice of meeting either Charade or someone else.