I went the other way, but the process will be similar. Since you didn't mention it, I assume at the present you do not any airplane ratings. You will have to get your commercial add-on first, as the ATP requires 250 PIC in airplanes. The commercial add-on requires 50 hrs in airplanes. The instrument rating requires 15 hours in airplanes. If you are current on instruments you will be bored out of your skull before you finish the 15 hours. However, since the commercial certificate requires at least 10 hours of instrument time, you can quickly finish that portion up. There are no additional written required for the commercial or instrument. The ATP however does require a written exam. You do get credit for your helicopter time. All you have to comply with is the times that say airplane.
I would suggest that you get an instructor that is dual rated. That way the instructor will understand where you will have problems. The ATP (A) written is an add-on written so it is quite a bit shorter. I would try to take it prior to August 14, 2014, as the requirements to take it will change quite dramatically on that date.
Good luck