I did not make this out of a pillowcase but the directions I followed were gathered from various sources from online, with directions on how to make a 'pillowcase dress.'
I did keep the tie to one long tie instead of the common two ( 1 in front and 1 in back, in casing), tied it to a ribbon on one side, and adjusting the material to fit properly over the body. I also stayed away from the elastic and just kept the casing only wide enough as the ribbon; this, and the fact that the ribbon was ribbed and the material being flannel, there would be little shifting.
I also used a serger to keep the material from fraying and sewing it down about 1/8" on the wrong side. Without a serger, it would be 1/8" folding the edges to the inside (wrong side of fabric), press, fold in again and sew for a clean edge.