Ruth ...here is a site that shows step by step how string quilts are put together...for their sample they are using all scraps.... what I usually do is choose a focus fabric and cut them all the same width for the centers of each block which creates the main lines going thru the quilt.
http://quilting.about.com/od/quiltpatternsprojects/ss/string_piecing_2.htm
String Piecing - Learn to Make a String Pieced Quilt - Learn How to String Piece