Sounds like we're on the same page

I would avoid heating the liner to get the faceplates on - with a 70*+ day and sun, the liner should be pliable enough to get the faceplates on. If you need a little more stretch, you can cut the center of the liner that you'll be removing from the steps (in other words, stretch it from the stair side NOT from the corners). Definitely have the bottom plate installed first!

Please let me know if you have more questions about this!