No. 1 on the list, take away the pain. Do not let her get hurt while learning.
I was taught to slide with the right leg extended so when you come up, you can face the infield. I teach either leg, and preferably the left leg extended so you can come up facing the direction the ball will be if missed.
Sit her on the ground, with her knees up around her chin. Have here (Quickly) extend one leg. That is probably the leg she will reach with, and tuck the other. Stay with that and it won't complicate things more.
Have her tuck the other foot with the ankle under the extended knee. (Her bent (figure 4) knee will be off of the ground).
Pick her up under the armpits (butt off of the ground, and this will allow the shin to go flat) and let her feel the points that she will brake with.
1. The heel of the extended foot.
2. The entire surface of the bent leg. (Protect the surface with a sliding
Or (older girls):
Put your foot against her extended foot, hold both of her hands, have her lean back, chin down, so she brings her rear off of the ground, this will allow the shin to go flat, and she can now feel the braking surface.
Pull her around by the hands letting her feel the surfaces she will be sliding
with, making sure she keeps the figure 4 with her bent leg, and not letting
it go under her rear. She needs to use this shin surface to slide properly.
(Take away the pain, so you may have to buy some knee pads or/and sliding
pads to protect the sliding surface.
After you have pulled her around the room or ground, have her stand in front
of you. (Preferably on a down hill grassy surface). Hold both of her hands
and have her put the bent leg in a figure 4. Then have her lean back so the
extended leg will slip under you (between your legs). Gently let her down
as she slides between your legs, and lay her hands on the ground extended
and over her head. A downhill slope makes the sliding easier and still provides
some friction for the braking mechanisms. A slip and slide is fun, and it
gets them used to getting down. However it promotes bad mechanics, (they
often roll as they are sliding) and because of the slipperiness, they don't
get used to breaking before the rear sets down.
First thing to check and focus on:
Make sure she keeps the figure 4 and doesn't let her bent leg slip so she is sitting on her foot. Make sure she doesn't bang her knee when she gets it closer to the ground. Make sure she keeps the reaching leg slightly flexed, so (1) she can use the heel to brake, and (2) when reaching the bag the leg/hip won't get jammed.
Next, have her stand Appx. 5 steps in front of you. She can walk fast toward you and as you grab her hands she can slide between your legs. And of course you can critique EVERY slide.
After she gets comfortable with this she can get back further and run. You can remind her that the faster she runs, when she reaches with one foot, the closer she will be toward the ground, and the more she will slide instead of fall vertically. As you will find out in a hurry, you can only do this between the legs method, with smaller girls.
You will be surprised how fast she will learn if you TAKE AWAY THE PAIN.
If the girl is bigger you can use two people to catch her as she comes between you. The coaches must move quickly and run with her as she goes down, or else she will fly past you as she slides, and you will drop her, and she will get hurt, and she will quit.
As she is coming toward you from the north the coach on the west side grabs her right hand with his right hand and places the left hand under the elbow. The coach on the east side grabs her left hand with his left hand and places his right hand under the elbow. As she goes by they gently lower her to the ground. She starts running faster, and faster until she feels comfortable about doing it herself. This is her decision. It is better to take your time now, even if it takes a half an hour, or an hour, or a day (etc) longer, than to loose the years ahead of her not sliding.
Another way that is not as effective to me, is to sit, knees up, and find out which leg they extend. Then while standing, have them put the other shin on the ground so when they sit down it will be in a figure four. Have them go from the knee to the rear, and then roll to their back, hands over the head going to the ground. Have them go from standing, to sitting and rolling, this way a few times. Then have them walk and on "Q" put their knee down (figure 4) then sit, then roll back and hands over the head. Keep having them walk faster and faster doing the same drill of sitting/sliding, until they feel comfortable going down. The faster they go, the more it will turn into a slide.
One thing to watch for is the knee going into the ground too hard. At first
they almost stick the knee vertically into the ground and it is just catching
the weight before sitting and rolling back. However as they walk/run faster
the knee/shin has to point forward, in a more horizontal position, so
it won't jam into the ground.