Well now you have a covered chair with tails. All you need to do is finish the raw edges of those tails and fold them to the back. Any fastener will do to keep them together.
Right now the back of your chair looks a little droopy. Those usewn gusset parts need attention. Fold a hem in the top edge which is the same depth as the seam allowance. An easy way to do it is to fold the loose end down and pull toward yourself on the same plane as the seat. The hem will automatically fold in.(↑ Sorry about this pic. I need three hands.)
I kept my gusset in one piece because I was draping the pattern at the same time as making the cover. Your gusset will have an end on each side. Fold over the hem on both top sides and across the center end. Finally. fold up an even hem all the way around the bottom edge of the cover. Topstitch or blind stitch in place.
Fold each flap around the back to the center wrapping one over the other to close the cover. Any fastener will do here…so I can’t decide. I’ve used twist pins until I choose buttons or snaps or maybe even a zipper.
CONGRATULATIONS ON COMPLETING A DEMANDING PROJECT!
Here are pictures of one finished chair…