Skipped steps 6-10; did step 5 so could lift top of display higher, then propped up. A photo of whole interior here would help (hard disk is top center). Used pliers holding T10 bit at right angle to unscrew hard disk holder. SSD installed loose (macOS and fan control app already loaded).

I followed this to replace hard disk with an SSD. These instructions and comments pretty much cover it. Only had to disconnect #7 cable and a little cable near the center of the system board; flipping/rotating bezel/screen avoided other disconnects. I used a permanent marker to label bezel holes "L" and "VL" (long and very long) to put those screws back properly.

My main problem: I expected a 2.5" hard disk but found a 3.5". Micro Center talked me into an IcyDock 2.5" --> 3.5" adapter, defective (ended up just taping the SSD down). Between that and not loading OS on the SSD before installing (basically: attach SSD via USB, boot while pressing command-R) meant I got to do this disassembly/assembly several times.

Worth it to buy a bottle of air to blow out dust, a 3.5" external enclosure (for the former internal hard disk), and maybe a CR2032? coin battery while you're at it. Group permissions on the hard disk's user files will be broken afterwards, easily fixed.