I have noticed that there is often something missing in the description of slave positions, which is how to assume the position in the first place.

I am sure that most Dom[me]s are like me and prefer their subs to appear as sexy and attractive as possible at all times, therefore movements should be smooth and graceful wherever possible.

As an example here is my opinion/preference on how a sub should normally move into the basic kneeling position (Gorean Nadu):

Before kneeling stand straight with the upper body presented in the same way as it will be when kneeling (back straight; shoulders back; head erect; etc.). Hands/arms whould be positioned according to circumstances, e.g. if not carrying anything I prefer forearms horizontal behind the back and back of one hand in the palm of the other (in this case they should be moved into their proper position on the thighs in sync with the rest of the body movements described below). Feet turned outwards, typically forming a right-angle - although you may find a more obtuse angle better if your intended kneeling position is widely spread.

If your sense of balance is good enough you may be able to shift your weight forward; lift your heels so you are standing on your toes; at the same time bending and spreading the knees which should (hopefully) descend to the floor in one smooth motion. Even after a great deal of practice you may not be able to achieve this however without risking toppling over - and if you are carrying anything such as spillable liquid it's probably not a good idea. A practical alternative is to move one foot backwards and lower down onto that knee while the other bends keeping the calf vertical (so that knee ends up near your armpit), then continue the movement by moving the other foot back near it's twin so that knee then descends onto the floor.

I recommend practice in front of a full length mirror with and without a glass of water until the movements appear smooth.

Happy kneeling
VV