I use the pre-treated fabric sheets you buy at JoAnns, I know some on here treat the fabric themselves to save a few dollars, but I don't want to work with the chemical. I guess it is like some on here quilt and some on here 'quilt by check.'
I make sure the fabric has time to cure after using my ink jet printer to print the picture (usually 24 hours) before I follow the heatsetting directions.