Identifying the two sizes of brackets
There are two frame sizes. One frame is wide and the other frame is thin. Make sure to attach the correct-sized parts to the corresponding frame size. (Use the wider bracket with the wider frame, the thinner bracket with the thinner frame.) The thinner bracket can be used on the wider frame pieces, so when it comes time to assemble the other side, it may appear as though the remaining brackets are the wrong size. It's just a matter of identifying the correct brackets to use with each frame piece.
The brackets will fit comfortably in the corresponding frame. Just tighten the screws to continue assembly. In the event the brackets do not appear to align, back out one screw slightly, just enough so that the bracket can wiggle. This allows the two frame sections to align properly so that both screws can be worked in.
Slide both sides together
Slide both assembled pieces of the frame together and leave at your desired width.