I would say from 4-6 months of age is a good time to have it done.
I've found with the buns I've had spayed that when they are done at an earlier age they recover faster.
When a bun goes in for a spay, they are put to sleep using a special mediacation, then a small incision is made into their tummy and their uterus and ovaries are removed. She is stitched up and then another medicine is given to wake her up.
The bunny normally receives a shot to relieve pain and pain meds to go home with.
I've had 10 girls spayed (mine and fosters), and I haven't had a problem.