I'm assembling a swing set, having issues keeping it square
If the issue is a flat place to put it, see Kyle B's answer.
If the issue is squaring the individual parts into one of these frames, cast your mind back to geometry, and make the diagonals equal length (i.e. get the screws into a section but not completely tight. Measure the diagonals and nudge the frame until they are equal, then tighten the screws, and check the diagonals again just in case.) This is typically much more accurate than using a square, though you can certainly use a square in the same manner (assemble loosely, nudge to square, tighten) and that may be close enough for this purpose.
Edit to add: Putting in one screw, adjusting the diagonals, and only THEN putting in the rest of the screws will work better if you are not using pre-drilled pilot holes for the screws. I was assuming that this being a kit it has predrilled holes, which should be close enough that having them all in but a bit loose should allow adjustment to square.