I use Sta-flo starch, 1/2 starch to 1/2 water, and dip the panel in that and let it drip (don't wring it out!). Then I block it, which is the same as others have been telling you: pulling and pinning it square and letting it dry.
It's not just the panels. There is a lot of "unsquare" fabric out there. I tried to make straight borders by ripping the fabric instead of cutting, on a fabric I thought I had squared. It's usually how I do it to get them straight. But trust me, even that doesn't always work. If you mess up your yardage making borders and they're wonky, it can get expensive!