I just did this recently... I'm no expert, but here's what I did.
Mine was 80% disintegrated, so I simply used my power washer to remove what was left. Once the surface had dried, I then used a product called Goof Off! and put it in a sprayer bottle and worked small sections with a plastic scraper. Spray on, let sit for a minute, scrape. You need to clean the scraper often. Repeat until 90% is gone. Then switch to a rag and soak it with Goof Off and remove the rest. I then cleaned the surface with mineral spirits. Probably took me an hour total.
I bought my pad from Authentic Classics, and was surprised at how stiff it was when it arrived. In hindsight, I would have left it in the sun for 10 mins to make it easier to work with. I started with the driver's side, and test fitted it with the backing paper still on. I had to do some minor trimming as mine was a retrofit, not restoration. Once satisfied that I knew where it needed to be placed, I used a razor blade to slice the backing paper into about 4 or 5 sections where I wanted the adhesive to be applied first, second, etc. I decided on applying bottom up, so peeled back the paper on the bottom section, then worked the pad into place carefully without letting it touch the chassis (the adhesive is incredibly sticky!). Once I got it past the hood release cable, I positioned the top to align, then slowly "smoothed" the bottom portion down and outward to the edge and worked the surface to ensure all areas were making good contact. I then removed adjacent paper backing pieces one by one, and smoothed down each section.
The other piece went down easier, but I had to cut a rectangle for my chassis ID tag. I made one mistake - below the coolant reservoir tank, there's a fat cable that I didn't notice earlier, and had to trim around it with the pad 75% affixed... should have done that during my test fit. Definitely a job I should have done long ago.
