I'll be making a new recovery, but the short version of how you do this:
- Go to fastboot and determine which slot you are on.
- Flash custom recovery to the current slot
- Reboot recovery and see that it behaves correctly
- Flash custom recovery to the inactive (other) slot
- Get update.upx, decrypt, unzip on desktop (no payload dumper needed).
- Put payload.bin & payload_properties.txt on a FAT-ish flash drive
- Reboot recovery
- adb shell start autoupdate
- Remove USB cable, plug in OTG/flash
- Drink a coffee, watch the little LED on the flash blink
- Put the flash in the PC and look at autolog.txt