Ragging and Frottage
If you want to use ragging on your walls, first prepare a 1:1 mixture of wallpaper paste and latex paint. To apply color, pour the paint mixture into a paint tray and dip scrunched-up fabric into the paint. If you want to use the technique of "ragging off," simply apply the paint mixture haphazardly (but with an even coat) to the wall. Then dab scrunched-up fabric onto the wall's surface to remove some color.
If the fabric gets too saturated with paint, be sure to take time to wring it out occasionally. Otherwise, you might find that your ragging pattern is sparser in some areas and more intense in others.
Frottage is similar to "ragging off." Apply an even layer of paint to the wall. Then, take paper or plastic that has been crinkled or scrunched, and place it onto the freshly painted wall surface. While the paper or plastic is still touching the wall, crinkle it further. This will aid in the pattern appearing smoother and more natural. If you find that you need more color, give the wall another layer of paint and repeat the process. Dab and crinkle the paper or plastic as necessary. You will find a method that works best for you and the look you are trying to achieve.