Here's how to do it: The moves in a crooked grind are very similar to those in a nosegrind, so it won't take long before you learn them. Perform the trick just as you would for a regular nosegrind, but go on a bigger angle. (It usually helps to loosen your front truck in order to make the board crooked) Grinding on an angle is the only difference, that's it. To come off the obstacle, pop a mini nollie, although many people find it easier to pop a small nollie shove it off the obstacle, so it all depends on personal preference.
This trick is really useful, and one of skateboarding's most popular tricks.