I'm no expert, but here's what I think....
First, separate her from the others (keeping her in the house probably would be best)
Clean the blood off with a damp paper towel
Then, when I get minor cuts, I put neosporin (no painkiller!! - one of the ingredients in the painkiller is toxic for chickens) on the cut... don't know if that's good for such a big spot, though... You probably would want to cover the spot (would a bandaid work?) to keep her from scratching at it.
Make sure she's well hydrated and she's eating...
Can she see out of that eye?
Also, reintroducing her to the flock might be hard.....