Basically, you need a piece of fabric the length of your hammock or tarp when it is rolled up and/or a tad longer if you prefer. Be sure to also include enough length for the end seams. On my last set, I used 13-inches wide by length of my OES tarp, plus 4 inches. Cut on a diagonal with small end at 4-inches and large end at 9-inches. This is pretty tight on small end - you might want to try 14" wide and cut 5" x 9". Finish ends with a flat seam and then overlay and sew on grosgrain leaving a channel on outer edge for inserting a plastic wire tie to give the ends a shape. Insert wire ties. Make sure wire ties are short enough that you can run the long edge seam across the ends without hitting the wire tie.
Overlay long edges and finish with a single straight stitch flat seam. This is the tricky part since you are sewing the diagonal. Keep the fabric tight or it will not be a smooth match along the full length of the skin. Next, overlay the grosgrain on the long edge and run a straight stitch as close to the inside edge as you are comfortable with. ( I first tried using an iron to get a nice edge on the fold of the grosgrain, but found out I could do just as well by keeping a tight hold on the fold as it fed into the thread injector. ) Then run another stitch further out toward the outer edge of the grosgrain. Be sure the grosgrain extends over the ends and oversew the closures at each end.
My sets are a little loose on the tarp and my WBBB when I pull them on, but it lets them expand a little after they are sleeved.
Hope this helps. Took me 3 tries to get a decent set - and I gave that one to my son for his HULBA. Practice makes almost perfect!