Found a quarter panel for my 63 starfire now how hard is it going to be to replace any help out there never done before.

 

Measure twice, cut once, comes to mind!
If you're replacing because of rust, cut where you need, - an inch for overlap, unless you're good at buttwelding!
If it's because of bent metal, pull it straight first, so the attatching parts line-up!
Pictures woud be good, but it's NOT a job for a novice, with all the welding/brazing/aligning!

 

I sent you a PM, get back to me if it's helpful
Steve