How to Upgrade Sony Xperia Z3 Compact (D5803) to Android 8.1?

How do you upgrade the [Sony Xperia Z3 Compact (D5803)](https://support.sonymobile.com/ca-en/xperiaz3compact/supportDocumentation/) to Android 8.1? There are a number of web pages that attempt to answer this question, but none of them provide every step along the way. These include: * https://forum.xda-developers.com/z3-compact/general/z3-compact-marshmallow-to-oreo-step-step-t3720857 * https://www.getdroidtips.com/omnirom-xperia-z3-compact-8-1-oreo/ * https://rootmygalaxy.net/install-android-8-1-oreo-on-sony-xperia-z3-with-carbonrom-cr-6-1/ * https://arnowelzel.de/en/carbonrom-on-the-sony-xperia-z3-compact * https://www.kbloghub.com/2018/02/root-install-twrp-recovery-on-sony-xperia-z3-compact.html * https://www.reddit.com/r/SonyXperia/comments/5vt6cs/flashing_marshmallow_concept_rom_on_a_sony_xperia/ They all skip steps or are missing key pieces of information that someone who is unfamiliar with the process would prevent from completing. A comprehensive, step-by-step guide for a complete novice would be helpful.