I was talking about stretchers the other night. I think I will continue in that vein by talking about squaring them up prior to stretching. There are several different ways of doing that. It is important that your stretchers be square. If they are not your pictures will not fir your frames, and That's a nuisance.
The standard method, I suppose, is to assemble them and using a tape measure, manipulate them until the dimensions between the opposite corners is equal. Once you have those two distances equal, Staple, screw, nail or ignore the corners together. If you fix then together (which I routinely do) you will be unable to use stretcher keys. Those are the triangular pieces of wood or plastic that are affixed to the back of a canvas when you buy it prestretched. I never use them because they knock the canvas out of square. If a canvas gets loose on the stretchers I pull out the staples on two sides and pull it taut again. Again if you knock the canvas out of square it won't fit the frame.
The easier way to square a canvas is with a drywall square. Here is one of those below.
They are made fore people hanging drywall, or sheetrock. They are about four feet long and you can rapidly push the stretchers into square using one. This in what I usually use.
Sometimes when you are traveling, you may not have either one. Find a steel doorframe, or window, or some other feature of a building that is liable to be square, assemble your stretchers and push them up against the inside dimension. Then turn the stretchers the other way and do that again. Usually you can get close enough to square that way.