I just had a thread about this exact same issue...here is a link...
Using Wrap and Zap. I did as stated above and soaked my WnZ in very hot water, then washed my fabric in hot water also. As of this writing though I have not made a bowl w/the preshrunk fabric. I want to quilt the pieces a bit heavier before assembling. I like the idea of practicing FMQ w/the bowls. Anyway, hope to get one done in the next couple of days and I will post new pics. Wish these tutorials would state that the bowls holders look awful if you don't preshrink, would save some disappointment.