Dear Unsure,
Get at least a 40 Fr colon tube. The ones they make these days aren't nearly as good as the ones from 10-20 years ago. They are much softer now. Finding a soft colon tube isn't a problem. It is easy to make it into the sigmoid colon. Soft colon tubes easily kink and fold back on themselves. I routinely get heavy, fairly stiff colon tubes 48Fr and larger (up to one inch in diameter) 24-36" inside. It takes practice and knowledge of anatomy--textbook anatomy and your own anatomic variations.
You will probably notice resistance at the end of the rectum going into the sigmoid. You will poke around and the tube won't go any farther, until it just pops through and you'll be there. If the tube folds back and goes the wrong way, and you keep feeding tube inside, the tip will just pop back out your anus. in which case, just pull it all out and start over. Never use anything like a drinking straw--they are sharp and will cut colonic tissue.
I like to do the entire insertion on my back. I can get the tube very far inside. Besides, from the sigmoid colon on to the cecum, the colon is basically against the abdominal wall and in front of all other organs, so being on your back means the other organs will not be pushing against the colon as you try to insert the tube. That's the reason colonoscopies are done with you on your back, with the exception of the initial insertion of 10-12" or so.
http://enemas4fun.zity.biz/forum/index.php?topic=2547.msg21345;topicseen#msg21345
The method I use to insert a colon tube is to be at least partially cleaned out first and be laying on your back--that is the position colonoscopy is performed in once you get to the transverse colon going on to the ascending colon and cecum. Then when inserting the tube, use lots of lubricant so the anus doesn't get sore. Then allow some water to flow in and close the clamp. Now to insert the tube, I like to use the "jerking off your husband" stroking method as the tube is gradually advanced. What I mean is keep the tube moving in and out in two to four (or whatever is best for you) inch strokes and keep advancing the tube. I never found just twisting the tube to be as effective as the "jerking off" method, which is actually more or less what gastroenterologists do when they insert a colonoscope. They inflate the colon with air, we will use the enema water. Periodically, depending on how the tube feels inside you, add more water and close the clamp again and advance the tube more. Keep doing this until all the way in. WARNING: the tube will not go in as far as you think. Unless you have an abnormally tortuous colon or the tube folds over on itself inside you, you won't get in five or six feet of tubing. When the colon is described as 5 to 6 feet long, that is how long it is stretched out on a table, not sitting inside your abdomen. Consider the colon to be like the slinky toy. You can stretch it out far, but when it is at rest, it isn't that long. To demonstrate my point, look at a chart showing how the colon lies inside the abdomen. Then, take a rubber tube and trace that out on your belly. You'll see there is no way you are getting five feet of tube inside you without its kinking and folding. Three feet is about the maximum you will get. DO NOT force it, especially as you get the end of the tube in to the lower right quadrant where the cecum is. If you have a heavy, stiff tube (which is really what I like because it is easier to feel inside you and "massage" to where you want it to go) you can puncture your colon, which will cause a life threatening infection or you can damage the ileocecal valve if you try to force it into the ileum.