It's really a good idea starting out to at least take some kind of class to get the basics on upholstery. I've been cleaning it since my first year back in 1987, but I've had a few close calls. I'm gonna set your mind at ease and make you worry a little bit at the same time because you shouldn't take doing upholstery lightly. The chances of liability doing upholstery are far more than cleaning carpet. You may clean for years and never have a problem, but then one day you walk in and begin to clean something that you don't realize the potential problem and it takes a big chunk out of your rear end! So here's my advice:
1. Take the class ASAP. Learn the basics of fiber ID and how the potential problems happen and how to avoid them.
2. DO NOT make it a practice to use carpet cleaning presprays on upholstery. Yes you can get away with it on some upholstery, but use it one time on the wrong upholstery and you may wind up buying it.
3. I personally think it's pretty safe just to use an acid rinse for everything and that's the way I've handled it for years. A good acid rinse can help eliminate and prevent some of the common things that can go wrong with upholstery cleaning. I use Matrix All Fiber, but there are others just as good.
4. Know that most cleaning labels are pretty useless although I have seen some improvement in recent years. Most of them have to do with what type of cleaning agent will cause that fabric to bleed.
5. In addition to extraction, I strongly suggest you learn how to use a good fabric shampoo and a sponge. I normally use this on naturals or natural/synthetic mix along with extraction with my acid rinse. This is something I can't overemphasize and I wished I'd learned it when I started out.
6. Following proper procedures with proper chemistry will eliminate or prevent most of the things we worry about such as bleeding, shrinking, browning, etc. I've only seen one couch really bleed in over 30 years, but fabrics with really deep reds or other colors are ALWAYS a concern.
7. Spotting can be a challenge, particularly on natural fibers. If in doubt play safe. It is better NOT to get a stain out that the customer caused than to remove the color that YOU CAUSED. Be cautious of using strong peroxide products on prints and naturals if you don't know what you're doing. Under-promise and over-deliver. You WILL NOT save them all and you will not remove every single stain although most furniture will turn out great some will be a real challenge.
8. Don't get it too wet in the first place and leave it as dry as you can. Especially important if cleaning naturals!!! Use a low moisture upholstery tool. There are several good ones out there that really help make the job a lot easier and that alone can really make a difference in how the final job turns out.