Maxivmus of MaxUI started to do a guide awhile ago but it hasn't been updated in awhile.

when creating an external edit the first thing you'll need is a .toc, it's pretty straight forward

This is how to use xml files for loading your files

We have a new way to hook into Tukui functions, you'll have to change things based on what you're wanting to edit but I'll be giving an example on...

as a reference:

/tukui c - tukui config menu
/tukui config - see above
/tukui test - show all unitframes
/tukui dt - start datatext module
/tukui dt reset - resets the datatexts to base values
/tukui dt resetgold - reset gold tooltip on gold datatext
/tukui move - enter move module to move available parts
/tukui reset - reset Tukui to standard config
/tukui install - start the installer
/tukui...

Not truely a guide but some smaller examples of Tukui plugins and a basic framework to start your own plugin or edit. These are all externally done like Tukz wants.
They were all made over the course of time by following other examples, copy pasting code, adjusting and learning step by step. Each plugin is small and uses some simple coding.

To start your own edit, there is a basic Tukui plugin...

Things to have in mind
Tukui & ElvUI are complete Blizzard ui replacements. This means that they already have Blizzard frames skinned, unitframes and actionbars premade, etc. So, addons like Bartender and Dominoes for Actionbars and Z-Perl Unit Frames or Shadowed Unit Frames will not work well (if not at all). On the other hand, addons like Bagsync, damage meters like Skada, Recount, Details,...