Well it's going to be slow at first no matter what. It's going to take you some time to build up some clientele. The way they do feedback will help you though if you get lots of good feedback, passers-by will see that and be more-likely to get a reading from you.
Just figure it up by the minute. I was thinking about easing into it by doing 1 hour a day for 5 days week. That's 1200 minutes a month (assuming I would get that starting out) at .45 a minute. That would equate to $540 a month for 20 hours of work for the month. Then of course, you'd have to keep your own taxes out, but it's really not bad if you keep at it and build yourself a reputation. Maybe after 6 months or so, you could raise your per-minute rate. I know my friend started out at $1 per minute, and 2 years later she worked herself up to $3.09 a minute now as her current rate. If you take 55% off of that, she's grossing about $80k a year but she works at it pretty regularly... and that figure probably does fluctuate depending on the business she gets, but if you are just thinking of doing it for some "extra" cash, then I don't see too much risk in it. It could be pretty much whatever you make it.