When assembling your frame in the first place, push the top and bottom rollers into the ends of the side bars and put the four bolts down into their holes and bang them home firmly so that they bite into the wood. Turn the frame over, feed a washer and a wing nut onto each bolt and turn these enough to keep in place but not to tighten just yet.
Your frame came with a very good illustration to show you how to attach your canvas and you need to follow that in every detail.
In other words, place your frame upside down with the wing nuts underneath; twist the rollers so that the grooves are visible and place your canvas upside down on top.
(I got both of these wrong to start with because I did not follow the photograph closely enough and I can report that it doesn’t work!)
Add clips one by one along the top roller by using the angled edge of the clip to push the canvas into the groove as in the photograph. Follow a line of holes – about half an inch in from the edge of the canvas – to keep it straight. Then do the same for the bottom roller.
Now take up the slack by winding the canvas in the direction of the arrows on the clips onto the rollers according to where you want to begin. I like to begin stitching at the top so I roll most of the canvas onto the bottom roller and just a little at the top to make sure it holds firmly. If you would rather begin in the middle, wind the rollers accordingly. The tension holds everything in place, so just turn it over, tighten the wing nuts and you are ready to stitch. Easy!