I used the key lock alot myself. Including a few illegal attempts (whoops).
Let me try to explain.
Have your partner post his left arm on your shoulder. With your right arm, grab your opponent's wrist. I normally set it up off of a russian. Once you pull the russian off, use your left arm to go under his arm and lock on your own wrist (key lock).
To throw, there are a few different ways. It is important (as with any throw to have a good set up). Pushing and pulling. The throw will be a belly to belly move where you essentially roll your opponent's arm into his own body. It is important to hit a good back arch and keep little space between your torso and his (with the arm in the middle). This will ensure you load him properly. Finish the toss by switching off or keep the keylock in a jap whizzer style pin.