The method of sucking water (or air) into the anus is difficult to describe. In order to understand it you must try it for yourself. The method is as follows: Exhale deeply. Hold your breath. Gently open the anal sphincters with your fingers. Do madhyama nauli2. Then, while retaining your breath and madhyama nauli, try to expand and pull up the abdomen.
This process creates a vacuum and sucks in water. As you draw in water, gently manipulate the anal muscles so that there is free flow through the anus. Some people can do this technique the first time; others require a lot of practice. It is very easy when you know how. If you have trouble sucking in water, then you are probably doing something incorrectly. You should contact a local yoga teacher who will be able to quickly point out your mistakes. If necessary, see a personal demonstration; this will be far more useful than one hundred pages of descriptions.