The first, and most important step is to have the key in position 1 or 2 (run or acc.)
If the key cannot be turned then the whole steering lock assy. needs to be busted out and replaced. Since you have got the key to turn, the procedure is relatively simple.
The ignition tumbler has a small hole in which you insert a suitable tool, a sturdy paper clip does work in a pinch. Turn the key slowly back and forth until the tool slips all of the way into the steering lock. This releases the black collar, which should turn counter clock wise. It turns stiff at first. Pliers may be required to turn this collar at first. Later cars, 202 and 124 chassis, the collar pulls straight out when released, and uses two tools. The factory tool is a single wire bent at the middle.
Reinstalling reguires patience to align the tumbler and the collar. Good luck!