I had 4 large panels that were not square and HAD to be because I was fracturing them. I basically beat them into submission!!
Wet them thoroughly with water/starch mix. Pin them out on a grid and force them into square. Then ... wait for them to dry.
My panels were about 2' wide and 3.5' long and I drew a grid on my carpet (it's those carpet squares with a very very low "loft" - almost flat) with chalk. I first pulled on opposite corners of the panel to stretch it into shape as much as I could, then I then used the grid to insert pins through the fabric and into the carpet to hold it in place - working the shortest opposite corners first and working my way evenly across the horizontal and vertical straights. I used LOTS of pins.
It did work but word of caution ... do not handle them too much once you pick them up. Do not iron them again. Sew them immediately.