It’s very annoying when parchment paper doesn’t lie flat! I’ve discovered the joy of buying folded parchment paper, it’s much easier to use. However, if you have the kind on the roll, lightly mist the baking sheet with cooking spray, then press the parchment on top. This creates a tacky surface so it will cling. Another trick is to crumple the paper into a ball, then smooth out and place in the pan. This softens the fibers to make it more flexible so it will lay easier.