I've never done it at home, but worked in billing years ago in an office. It's hard to imagine it being done at home, but apparently it is. Anyway, careerstep.com offers courses in this, so you can check them out. I went there for medical transcription training and had good success with it.
I am taking a medical transcription course through Allied Schools (www.alliedschools.com) and I know they have a medical billing course. You said you have experience though, so you might not need more school. OK, guess that didn't really help... Good luck to you!
The only way you can do billing from home is to be in an office doing medical billing first. Your boss would have to make the decision if he would allow you to do it from home. Some do. Otherwise you have to open your own billing buisness from home. I took the course for medical billing from The Career Institute and tried to do this from home. Opening your own billing buisness is not only expensive, time consuming, it's hard. I tried, put in about $2000 and just couldn't compete with the large billing compaies that the doctors in my area use. I have my own medical benefits business at home now and love it. It was only $295 (2 years ago) to get started and it's very easy. Now you can get started for only $95. If you're interested or if you have any questions about the medical billing, let me know!