I would wash and dry the panel before doing anything else. Because the fabrics are rolled mechanically after printing, the cotton warp and weft can be distorted post-printing.
If the panel is still warped, you can try a gentle squaring up as in this video, which someone else posted too:
https://www.youtube.com/watch?v=ZGE2B5jcsms
I would be less worried about being exactly on the straight of grain, but try a good machine wash and hot dry first.