The following pages document the steps required to flash a Custom ROM on a phone of a particular manufacturer. The pages also include how to flash a phone back to the manufacturer's stock OS.
Flashing Google Pixel 2/3/3XL/3a to CalyxOS and back to stock OS.
Flashing Samsung S4 GT-I9509 to LineageOS and back to stock OS.
Flashing OnePlus 6T to postmarketOS and back to stock OS.