My guess is that the forward canvas is already complete and installed on your boat.
It looks like the camper canvas adds a large rectangular stainless hoop, w/ curved corners, attached to the deck. Attached to that large hoop is a smaller hoop, angled forward, perpendicular to the large hoop.
There is probably a fabric strap in each rear corner, attached to the camper interior, to hold the rear down to the deck when the sides are removed.
The Sailrite site sells everything required to complete this installation. They also offer terrific technical support.
http://www.sailrite.com/All-Hardware/Boat-Hardware/Frames-and-Tubing
The trick is going to be to get the tubing legs the correct length so the canvas is tight.
If this were me, I would make the large hoop a little long, install the canvas and the hoop w/o screwing it to the deck, then measure the lengths and the angles. I would then use geometry to calculate the correct length for the large hoop.
The small hoop length is correct when the two tubes are perpendicular. Small adjustments are possible by sliding the fitting up or down on the large hoop.