Nancy, for best results, you will want to mordant your cotton with aluminum acetate and then dip it into a chalk or wheat-germ bath. Maiwa Textiles near Vancouver is a wonderful supplier of natural dyes and has a free download with excellent instructions on how to succeed with them. Go to Maiwa.com, then click on "Natural Dyes," then click on the first link "About Natural Dyes." At the end of the third paragraph, there is a link to this guide, a downloadable PDF called "Natural Dye Info Sheet."
Please share photos of your results with the group! We all need color these days ;o)