In terms of general difficulty, about a . No special tools mandatory, though they recommend a hair dryer, and it shouldn't take 5 minutes to do it. It was more difficult than other skins I've applied in the past because the one I applied here didn't cover the glass section, meaning I had fewer 'landmarks' against which to set the original position.
What I found worked well was to first line it up to center the FPS, then level the edge against the bottom of the shade (it's a perfect cut). From there it only took the tiniest adjustment to make sure the G logo was perfectly centered, and from that point on it was trivial.
Slickwraps does produce video guides for applying their skins to specific phones - I'm sure they'll have an easier way to accurately apply in the guide for the Pixel 2 / 2 XL - but this worked pretty well with minimal fuss.