At this step, if you mess up anything with these 2 cables, especially when plugging them back in, even if you think you’re following the instructions here to a T, then you’re screwed.
I would highly, highly, highly recommend AGAINST doing this at home. I followed these instructions to a T, and still somehow managed to have the screen not be able to turn on after the hard drive replacement. Then, once you’ve opened your Mac yourself, Apple will refuse to fix anything on the computer, and even other third party, Apple-authorized repair facility will refuse to repair it for you, and you will have to locate a non-authorized Apple repair facility. There, I am having to pay ~$600 to fix things that got screwed up, even though I followed the instructions here to a T, step by step, and did everything slowly and methodically.
Beware, if you’ve never done this before, that be locked out of ever being able to use an Apple repair facility, or Apple-authorized repair facility, if you mess things up here now.