I did my own, not hard at all. One trick is to slide the covers on over plastic bags, helps them slide into position rather than drag & hang up on the rubber buns. And don't forget to retrieve the listing rods from the old covers to insert in the new. Yes make sure they are warm & pliable, & take your time, it's a wrestling job.
Also, take a good look at the frames to see where bits of trim screw on before you start fitting the covers, the holes can be hard to locate through the fabric later.
There are how-to videos on-line, & written manuals in Mustang resto books to guide you through.