Kneeling is a compromise between time and the need to get low for a shot. It is very quick to get into and out of, which is useful for combat and competition, but I have found it lacking in stability as only one elbow is supported. A better compromise, in my opinion, is the squatting position. However, kneeling is extremely useful if you are able to make use of terrain to support the rifle.
In any case, there are two primary variants of the traditional kneeling position: high kneel, and low kneel. Which one you use will likely depend on your flexibility and comfort. Both variants use the same support side leg and elbow position, and really only change based upon the position of the shooting side foot.
High kneeling is the faster of the two. In a high kneel, the support side elbow is rested on top of the support side knee (or, rather, just in front of it while using the flat of the elbow). There should be a solid forward lean on the part of the shooter. The firing side elbow will be without support, so just keep it somewhere comfortable.
The firing side foot, in a high kneel, should have the toes planted into the ground, and the heel planted under the shooter’s butt. There is some debate between placing the heel directly under the base of the spine, or placing it under the firing side “cheek.”
Notice in the photo that my right foot is straight, rather than the toes rolled under.
This offers me better stability by allowing me to use the tough shank of my boot to support my weight. It is a pretty natural position to assume, as it really only takes stepping forward with my support leg and dropping right into the kneel.
Some people roll their toes under instead of keeping their foot straight. This method is also quick and natural to assume, but I also find that the “springiness” makes the position more unstable for me. I also find it more painful, but that has more to do with a toe sprain I suffered last year.
In a low kneel, the shooter assumes the same position as the high kneel, except the foot will be laid horizontal rather than vertical. The shooter then rests on the side of the foot. A variation on this position is to lay the top of the foot flat to the ground, and sit on the heel. In either case, the goal should be to place the foot in a position that it does not want to roll backwards with recoil forces or as the wind pushes you around.
To be honest, I’ve not had the best luck with kneeling in the traditional sense. I find it very useful when I can rest the rifle on a barrier of some sort, or of the target is of sufficient size. But when the wind is blowing, or I’m out of breath, I find kneeling to just be too unstable.