I did it last week. We were out of power for about 30 hours. But I actually have a degree in Computer Engineering, and I did it with a friend who is a professional Electrician.

It is indeed EXTREMELY dangerous. If you don't know what you're doing, or make a mistake, best case scenario, you fry your generator. Worst case, you electrocute a lineman from the power company, who isn't expecting lines to be live when there's an outage, because yes, if you feed power into your house, that will flow OUT of your house onto the main lines (to some extent), if you let it. You could end up trying to power your whole block on your little gas generator.

We made sure both the indoor and outdoor main power shutoffs for the house were turned off, as well as all breakers. Then we unplugged the oven, and used that for the feed from the generator. Then we gradually re-activated breakers so as not to add too much load to the generator at once. Ultimately, we were able to run the whole house, except for the AC compressor, which the generator actually would have had enough power to run, but not to kick-start.

The proper way to hook up a generator to feed your house is to install an "inlet" which is both nominatively and physically the opposite of an outlet: instead of holes going into a box, you have prongs sticking out of a box. Generally, it'll be one of the big fat 4-pronged round cables, like what your oven might use. That'll feed down to a large double-breaker, in the top-right slot of your breaker panel. That breaker stays off until you want to run a generator, and, to meet code, you have to also install a special bracket that prevents you from turning this breaker off without turning off the primary feed for the whole house. Still kinda dangerous, but they make those brackets surprisingly foolproof.