I had someone I worked with send me a request to like her realtor page on FB. I liked it and then sent her a message like "Hey, I'm interested in getting into real estate photography and was wondering if you needed a photographer for your listings." I explained that I was new to this genre, but not new to photography, so my starting rate would be cheaper and would get a little more expensive as I got better. At first they said no, but then a few days later asked for my rates. They immediately sent me two properties to shoot that weekend and I shot 4 properties in 10 days for them so far.
The most valuable commodity to a realtor is time. They would prefer you have the photos to them faster much more than them be perfect. I try to deliver them within 24 hours of shooting the property, so you need to be able to shoot fast and edit fast. Communicate with them a lot and be prepared to bend your schedule to work with theirs. They have to make it work with their schedule and also the seller's so it can be difficult to make it all line up. Being reliable and punctual will get you a long way with these guys.
Real estate will not make you rich unless you are shooting a lot of properties per week, but it is quick cash and that's super nice. Most people charge $50-200 per house depending on your area, so I would plan to work for the low end until you get good (I'm at $100/house right now).