"Remember, don't remove the old plastic until you have the new one sewn in".
Why? I unfortunately put my hand through my dodger window the beginning of this year and we plan to attempt to repair it ourselves. I would have thought you would take the old out, use it as a template when you cut the new piece and then sew it in. Since your advice is contrary to my intuition, I thought I'd ask for the rationale behind it so I can learn.
"To reach the port of heaven, we must sail sometimes with the wind and sometimes against it - but we must sail, and not drift, nor lie at anchor.”
- Oliver Wendel Holmes