The bowel must first be thoroughly cleaned before a colonoscopy. This is done either the day before or the same morning as your colonoscopy. You will be asked to drink medication that would make you have diarrhoea to pass out all the stools inside.

The colonoscope is inserted through the anus and advanced to the portion of the colon where the small intestine enters. Your doctor will remove any polyps or take biopsies if necessary.  

        A colonoscope and its view of the colon
        
        
The entire procedure usually takes less than 15 minutes. There is little pain, though mild sedation is given when necessary to relieve anxiety and discomfort. Most patients can resume their regular diet later in the day.
 
Why not a Barium enema?

Colonoscopy is more accurate than x-ray exam of colon to detect small polyps or early cancers. It also allows removal of these polyps, which is important in the prevention of colon cancer.
 
What are the risks of colonoscopy?

Colonoscopy is a very safe procedure. There is a less than 0.5 percent risk of damage to the wall of the colon, including perforation.