I've seen them mostly made with white backer rod. I have a bunch of blue rod (just to be different). I cut the backer rod into pieces 3/4" to 1". In the top 1/3 or so of the rod string yarn (2 different colors, making a cross) through the backer at a right angle (to the length of the backer). I use white and red, white and orange, white and chartruese, etc. The yarn should not stick out too far (1/2" or so..)

They seem to make a large silouette in the water so I tend to use them when there is 2 - 4 feet visibility. I squirt shrimp oil on the yarn and also use a shrimp tail on the hook. The rags also have a lot of bouyancy so my leaders aren't real long (18" - 24")

Then again, I don't catch many fish, so take this advice with some caution...