First, I measured 4 corner from the top of the bench to the floor, adding 4 inches at the bottom for a hem, and an inch to be stapled to the top. I machine sewed the hem since the fabric ( a linen blend) was coarse enough to hide the stitching). It was also thick enough no backing was needed. If using a finer fabric, I would have needed to use lining or interfacing and hand sewed the hem. If you don't sew, Stitch-Witch or a similar fusible iron-on tape could be used. Since the sides will not be seen, I didn't hem in order to reduce bulk. After hemming, I used a staple gun to attach to the frame.
I next measured the sides of the bench. In addition to adding 5 inches for hem and stapling, I added 2 inches to each side for a side hem. I used fusible tape to avoid a stitch line. I then stapled the sides in the same manner as the corners, placing them on top of the corners.
I then applied the short sides in the same manner, positioning them so that they met the front and back.
I then stretched the fabric over the seat, stapling in place. For added definition, I used self cording. If you don't want to go to the trouble of making cording, cording can be purchased. I then screwed the top back into place.
Sorry, I don't have a better finished pic, but before I could photograph it in place in the client's home, I developed camera issues, and I haven't had a chance to go back their. I have a more involved upholstery project to show in the near futuer.