I cut my own curly hair, although i only have experience with cutting it short (i'm currently growing it out). But in my experience of having long curly hair, it's far more forgiving long than short, so in theory that should, if anything, be easier (in theory!).
I did a fair bit of research through google before jumping in, so that could help. But i'll pass along the main things i've picked up.
Since your curly hair looks completely different when it dries than when it's wet, do not cut it while it's wet! Although, with a grain of salt - i've actually had the most luck cutting it when it's about halfway dry from a shower, that way the wet kind of holds it together and keeps the frizz factor from getting in the way, but it's dry enough that the curl is really showing.
Which brings us to cutting with the curls. Basically, this is the good alternative to a nice, even haircut - because a nice, even (straight-hair-style) haircut will look okay...the day you do it. But as soon as you touch it, or certainly when you wash and dry it again, the curls will completely re-arrange themselves into...usually, a hot mess. So, the basic idea of cutting with the curl is that your hair naturally falls into "chunks" that curl together (for me, they vary from very small (like 1/16" or less) to solid 1/4 or 1/2-inch pieces), and you are focusing on cutting each of these chunks of curl separately so that they hold a...complete curl. Sorry, that's really hard to explain, but your curls will have a pattern (rings, or S's or something), and you want to not cut the pattern in the middle - that's what makes the curls just stick out all over the place - you want to let the curls kind of "finish curling" and cut there.
Sorry, that's a pretty confusing description, but knowing nothing about hair or haircutting, that's the best i can do to explain it. I hope it make a little sense, or at the least it might give you some fodder for a google search =).
The other things i've found helpful:
Remembering that my hair is curly, so while wrong looks awful, "right" is a huge spectrum, because my hair looks different every day - in that respect, it's fairly forgiving of haircuts.
Getting the spouse (or whoever) to help clean up the back. Somehow there's always a hair or two (or an inch) that you miss.
If your cutting it yourself, you can clean it up anytime! No deciding by your budget whether it's "haircut time". Plus, with my curly hair, sometimes it seems like one single curl has grown all out of whack, and all you actually need is to trim the wild hair, not get a full cut.
Hope that helps a bit. (P.S. it will be absolutely terrifying the first time (or 12), but remember that the worst thing that can happen is that you end up going for that professional haircut that you'd have been getting anyway if you hadn't tried).