This can be made to fit either sizes of the tissues now available.
Start with three pieces of fabric, either 6 x 6 or 6 x 7. You will need a back, middle, and front. The thickness of the middle will depend on whether you want to do a bit of embroidery or machine quilting on it. And if that is the case, I would make it an inch or two larger in each direction. But the one I made is just simple, so I used a piece of sheet inside.
On the 6″ sides, I put a piece of bias tape to enclose the edges.
Then I folded it in half, found the center
and folded each side into the center. I used pins, but one could just press it.
Using a half inch seam, sew, across the end, backstitching at each end and over the bias tape.
Trim the edge beneath the bias tape and turn to the right side. Be sure to poke the corners out.If you prefer not to add the bias tape, you can layer them, outside face up, backside face down, and put the inside on top. Sew down the 6″ side and turn to right, between the inside and outside pieces. Top stitch 1/2″ from seam