I gave up cleaning curtains well over ten years ago, although I still have the capability.
The use of water based cleaning agents should be avoided because of the potential for shrinkage. OMS is the most commonly used agent, and you will need a solvent capable extractor. I have a couple of Kleenrite machines, and I believe US Products make a similar unit.
Cleaning curtains is not a particularly pleasant task, as you are generally operating above shoulder height, and inevitably the OMS will run down your arm, causing burns, even with the use of gloves.
You will also come up against sun damaged curtains, which can fall apart during cleaning.
As solvents and water don't mix, unless you have a dedicated extractor, you will need to flush the machine after use......a PITA exercise.
I never made any money cleaning curtains...most aren't prepared to pay the money you will need to make it worthwhile. But, I guess someone out there will make a profit from it.
I hope that helps, Clive.
Cheers,
Jim