Getting the digitizer cable threaded without having it fold or kink is, without question, the single hardest part of this whole repair. Everything else requires just being meticulous, measuring twice and cutting once, and making sure you use a reasonably light touch when detaching contacts, etc.
And after 37 steps (o hai Hitchcock!) you feel like you're home free.
But the digitizer cable is the last henchman between you and the Bond villain, the last lock protecting the treasure.
After you've threaded it through, pull GENTLY to make sure it extends fully (both cables should be about the same length), and check on the underside to make sure there's no fold. This is where a mm or two makes ALL THE DIFFERENCE. Keep checking it occasionally as you screw down the screen, because you don't want to have to go back after you've installed the logic board.
And I've replaced with both iFixit kits and 3rd party screens. The cable is always an issue-- but with patience they DO extend far enough.