Agreed about the suction cups. An extra pair of hands might be good here as well.
On the cards I got with my iFixit kit, the blue part was way bigger than 9.5mm — closer to 20mm!
I made a line 9mm from the edge of each of my cards and used that as a guide.
It still took me a solid half-hour easily to go along all the edges and get them as clean as possible. I had to go over some sections a few times. This might have been overkill, but I really really didn’t want to go through all this and then find that the new adhesive strips weren’t holding because there was old gunk I hadn’t removed.
To add on to the rubbing alcohol thing for removing residue, the alcohol doesn’t actually dissolve the adhesive residue, but softens it and makes it easier to get up. But you still have to actually remove it. I found that once you peel up the big pieces of adhesive, you’re still gonna have a some sticky residue. Here’s what worked for me:
1) You want to go section by section, using a clean piece of non-linty cloth wet with alcohol to get the glue residue nice and wet. As it says in the instructions, don’t wipe back and forth, but wipe the residue along in one direction instead of just spreading it around.
2) The plastic spudger that came with your repair kit (or the end of a credit card or something plastic that won’t cause damage) can be used as a little trowel to push the adhesive along and gather it up into a little booger-like hunk of goo.
3) At that point, you can wipe it off with a clean, dry piece of cloth. Why dry? Because it won’t really stick to the alcohol-soaked one but will stick to a dry cloth.