One sitting? I hope by your use of that phrase you are not suggesting sitting on the toilet and attempting to take a full bag (2 quarts), or even a half a bag!
Enemas give best results when taken laying down in a comfortable position, preferably on the bed with a pad underneath. Many people find the Sims position to be the most comfortable, especially during pregnancy. Lie down on your left side with your left leg straight and your right knee flexed. You should be partially over on your tummy with your left arm behind your back and your right hand in front of your face or under your pillow.
The most popular position though is on your back with your knees pulled up. Also used frequently is the knee chest position where you start on all fours, and then rest your forward weight on your chest with your face turned to the side on the bed or on an exercise pad or something on the floor to make it comfortable. Because the rectal nozzle tends to slip out in this position, it is best used when you have a helper who can hold the bag and keep the rectal tube in place. This is also a comfortable position during pregnancy, and is probably the most comfortable position when taking a large volume enema.