Base running - Sliding
A good way to teach a bent leg slide is to teach it the way you teach many skills, in steps.

Checkpoints for each step:
Unless you're stealing second, don't slide head first into second base. The chances of being stepped on and injured are too great. For younger players, the head first slide is not recommended for anything other than diving back to first. A head first slide puts you in a vulnerable position where it is easy to injure your hands, shoulders, and head. A good bent leg slide straight into the base is the best and safest way to get to the bag, protect yourself, and break up a double play.
Go to any youth baseball game and you'll see players who have scrapes all over their legs from sliding. Player's mistakenly think that a bent leg slide is performed by sliding on the side of the leg. The proper way is to slide on your butt. Practice sliding in grass and check the stains on your pants. If they are on the side of your leg, you're not sliding properly. Another common problem in sliding is putting your hand down during the slide. This is often the cause of jammed fingers, wrists, and sometimes shoulders. Get in the habit of sliding with your hands up. If you can remember to slide on your butt with your hands up, you will be sliding pain free from that point on.