Use by default backslash (\) to sit. Hold to sit on the ground. You can still move a bit while sitting to position yourself over an object. Use left bracket ([) or right bracket (]) to turn your head. Use the same key to return to the default position. Shift will slow you down for precise positioning. Use Dash (Space) to move your position upward and Guard (Default: C) to move your position downward.
If you have the 'input-api' mod, you will be able to rebind the keys in-game.