First off I don't really worry about specific measurements in the beginning. I just place the diaper on the fabric with ends extended either end. Place right side of fabric to the back side of the diaper with lower edge of diaper and edge of fabric even. Stitch approximately 3/8 inch -- could be a little more just an even seam allowance.
Next trim sides of fabric 1/2 inch wider than diaper and the width of the border you desire plus 1/2 inch. Press 1/2 inch hems on sides and lower edge.
Then press fabric border toward diaper at seam where fabric is joined to diaper.
If you wish you can add rick rack or pom pom trim before top stitching. Other wise just top stitch around the fabric .
You can also add machine embroidery to the burps or leave off the fabric border and just add rick rack to the lower edge.
enjoy and thanks for stopping by,