I would start with starching the piece really well. The way I square up fabric is is take the piece, holding both selvedges together, and then look at the fold. If it's not even, shift your fingers side to side to get the fold to hand evenly. When it's even all the way across, I lay it down on the ironing board and gently give it a crease. They I take the creased edge even with the other edge and do the same thing. Make sure the fold is even across and not wonky. You will know what I mean when you look at it. Then I iron another crease on that fold. I then make a cut with my rotary cutter to clean up one edge. Leah Day has a YouTube video on this so go watch her and you will understand better.
Good luck and I am sure others will be by to give you advice as well. Welcome to the board. Gina