Lining pans with parchment paper can be somewhat of a PITA, admittedly!
I'll show you a really easy and fast way to do this with a minimum of scissor-action.
My brownie pan here is about 5 cm tall, so I need a piece of parchment about that much bigger than the pan, all the way around.
Mark the corners with a pencil.
Cut the paper as below - from the corner of the paper to the corner of the pencil mark.
Flip the paper over before you fit it into the pan - so the pencil mark is on the underside (I don't want graphite in my brownies!).
Press the paper into the pan, lining up the pencil marks in the corners. The cut flaps should straighten themselves out and overlap each other.
And that's that! A fast, easy-peasy way to line your square and rectangular pans.