Considering the iPhone 7 is water resistant, just hoover your bathroom, get rid of that dust, then run a hot shower for a good 10-15 mins with no windows/circulation to get it nice and steamy, slather the phone in a slightly soapy water mix (you don't really want bubbles as such, just a tiny drop in a sink worth), dip the skin in to the soapy water and get it as close as you can to lining up correctly, squeegee out the liquid, start to get the corners sort of sticking then stretch and wrap the phone in clingfilm with the corners in place to keep applying pressure over night.
If the skin is slipping around too much when trying to line it up then squeegee a little of the liquid out, if you feel it's got too dry then just peel it off before it properly sticks and start again.
Peel the clingfilm off the next day, should be nice and tight, and hopefully thanks to the vacuuming and steam there should be little to no dust under the skin. Don't panic if it still looks a little misty or bubbley the next day, they'll slowly go away over the next few days as the phone heat cycles etc. If it doesn't start to show signs of going away after 48 hours then either you used a little too much soap or it had some other ingredients in.
Used to do this back on the 3G/3GS and 4, obviously I was a little more careful to not get any liquid them, which is why the iPhone 7 should be pretty easy! Although I stopped using them before the fancy corners came in, I used to install what they now call 'Simple' I think?