Here’s an easy way to cut just the right size of parchment paper for a pan, and for getting it to lay flat.
Place the pan on top of a large piece of parchment paper and use a pencil to trace around the bottom. (Most pens will not write on the paper very well.)
It’s even easier if you can trace the pan in one continuous line, without moving the pan all over the place and making a mess of things. I never was much of an artist.
Cut just inside the traced line for the right shape/size of the pan.
The parchment may curl up and not lay flat, as parchment is prone to doing.
To fix this, crinkle up the paper into a ball.
Now open it up, smooth it out and it will lay flat in the pan.