Using a staple gun or upholstery tacks, fasten the top fabric and dust cover to the frame. Pull the fabric tight before fastening. You don't need to attach them separately; one staple or tack through both fabrics will do.
In retrospect, staples are the better choice because they are easier to remove if you ever need to move the box spring again. I used tacks because they were handy at the time.
Once you're finished tacking the fabric in place, you're done! Enjoy your bed!