When I get panels that need squaring, I draw lines on my homemade ironing board cover, soak the back of the panel with starch or Best Press, pin the panel to the board using the lines to square it all up and let it dry on the board.
I use a lot of pins and I don't pull anything *tight,* just smooth and even.
Take the pins out and press and measure to make sure that you have it even. If not, you can re-do it until it's square and ready to cut.
Some of my lines are drawn with a Sharpie - which apparently wasn't a good idea because the Sharpie bled into a pale grey fabric that was soaked with starch, the other day. But, usually I use Frixion pens to mark the board.