I am severely constipated, and my doctor recommended that I clean my colon completely tomorrow with miralax (polyethylene glycol). How can I do that?

Mix with water. Miralax (polyethylene glycol) is an over the counter medication - you buy at your local pharmacy. Mix 17 grams of power - one tablespoon - with 16 oz of water and drink slowly. If your doctor wants you to do a stronger dose, this is not dangerous. You simply double up on the dosages above. For colonoscopies, that do a full cleanse, it is four of the above doses over the course of the day.