The main thing is to be comfortable. As mentioned in my book, I recommend that you lie on your back with your head raised and supported by a pillow or two. This will allow more blood to move the liver area. Most people are more relaxed in this position than they are when they pull the knees toward the head. Remember, the liver spans from the right side to the left. Keeping the liver in its normal position can be an advantage. Lying on the right side and putting pressure on the liver doesn't mean that more stones are going to come out. Whichever position you prefer, just make sure not to strain but be as relaxed as possible. This allows for easy passage of the stones through the bile ducts.
It is good to drink some water before taking the first dose of Epsom salts. You can drink water throughout the morning, except during the first 20 minutes or so after taking Epsom salt. In fact, it is important to drink water during these morning hours.