I would start in the center and work outward...Also be sure to wash the sheet first because they tend to shrink more than fabric...You can buy 120 inch wide white or muslin on the bolt...You can buy as much length as you want..It would be better for quilting...I also do machine embroidery...You could buy the same for the front and back, or buy some of the quilt-back wide fabrics....
But with quilting, it is really up to the individual...It is your quilt, make it as you like.....
Would love to see pictures when you are done or send some pics out as you finish each block...I am sure it will be beautiful....