You grab her from the roost at night, bundle her up in a towel with just her head sticking out, sit yourself down while positioning her on her side on your lap, the bad eye up for easy reach.
Then wash her eye using lots of saline eye wash and apply some terramycin eye ointment or similar.
Could be an insect sting. Do your chickens have access to fruit trees?
You could try some antihistamine (benadryl) and see if the swelling will go down.