To begin, I piece together some fabric fto make a wide backing, then trim it to make sure it is 'square'.
I notch the center of the top and bottom edge of the backing fabric, which is loaded on the frame WRONG SIDE UP.
Then I align the center notch on the bottom edge with the center marking on the canvas leader for the backing...the lower of the two front poles. I am using corsage pins to pin the backing to the canvas leader, putting my pins parallel to- and equidistant from- the edge.
Both sides should be the same distance from the center. I found it easier to pin the center first, then measure out to the place on the canvas leader where the side of the backing should be and pin that, then pin from the outside in towards the center; then repeat on the other side. This way, I am assured that each side will be the same. (I plan to make some new leaders with a tape measure sewn right on, but until then, I use my measuring tape to be sure I haven't pulled or stretched either side when pinning.)
Once the bottom edge of the backing is pinned across the entire edge, I smooth out the backing fabric and begin to roll it onto the pole, using my hands to smooth out from the center as I roll, so that the backing fabric will not twist or skew.
I stop rolling before the entire backing is rolled up, because not it is time to attach the other end of the backing fabric to the take-up roller at the back.
Again, the center notch on the edge of the backing is matched and pinned to the center mark on the canvas leader of the take-up roller.
I measure out to the half-width place and pin the corner, then work back toward the center.
Once the entire upper edge of the backing fabric has been pinned to the canvas leader of the take-up roller, I can finish rolling the backing fabric on the lower front roller.
You can see the seams where I pieced the backing...I hope they don't pose a problem for me!
Not pretty but it works!
Using the basting stitch, I baste the top edge of the batting in place...the 'channel locks' enable me to move perfectly straight to the side, giving me a straight line of stitching.
I use a pin placed vertically to indicate the width on each side...pinning the edge at the center then at the side and working my way back to center.