These can be hot or cold baths in which the water line reaches the hips, womb and ovaries, but stays below the kidneys so as not to chill them if the water is cold.
The tummy button is a good guide level; you may need to add some more water or let some out. Once you are in the water swing both of your legs over the side of the bath, or alternatively do not allow them in the water initially lowering yourself into the water over the side of the bath. Whether the water is hot or cold depends on the problem and the individual situation.
Sitz baths focus on encouraging circulation, increasing oxygen flow and relieving any congestion or stagnation there might be in the lower abdominal area. With cold sitz baths, where it may be difficult to stay in the water for any length of time, shout or scream, as this will relieve the shock and numbing effect of the cold water. Wear a jumper and socks if the bathroom itself is cold. Begin by staying in the water for one minute and progress to 5 minutes as you get used to it.