Rag rolling or "ragging" creates dramatic texture. Use softer, more closely related colors when ragging, as contrasting colors create too bold an effect.
Apply your base coat and allow to dry. Pour a small amount of your second color into a roller pan and take a piece of cheese cloth, old diapers, whatever, as long as you use the same type of rag for the entire job so the texture remains the same. A piece about 2 feet square is just right. Dip the rag into the paint and wring it out, rolling it into a loose rope like tube , much like if you were wringing out a wet "T" shirt. Take the rag and starting at the top of the wall, roll it down the wall from top to bottom.

Keep plenty of rags on hand and change often to avoid paint build-up.
Use both hands, when rolling the rag downward, changing the amount of pressure you apply to give a more varied texture.

Variations:
Your second color can be mixed with clear glaze giving it a more transparent effect, experiment and remember the mixture for the one you like.
Start with 2 parts paint and 1 part glaze.
Try "dabbing" the rag instead of rolling. Just bunch it into a big ball and pounce it over a small area. Turning the rag and your hand prevents a pattern which is what you want.

Ragging Off:
Ragging off is very similar to rag-rolling. Apply basecoat and when dry apply a full coat of the second color onto a given area say 3-4' square, then lightly dab ( or rag roll) at the wet topcoat with a clean, bunched-up rag. The more you dab, the more topcoat you will remove. Blend out any obvious edges between sections.

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