My method is simple, I analyzed all the codecs in
CadEditor, and nowadays I build new codecs based on the existing ones, for example
Robocop's graphics, they work the same as
Captain America's...
Edit: Settings_Robocop_1.cs I created it before I knew that codecs existed for this game, so these include some notes as reminders that briefly explain the function of each part of the script, so if this topic interests you, you may find it useful.
About your other question, there are several simpler ways to work, for example editing the screen visually with
DjinnMapper.
Although personally I prefer to create a mockup in an image editor, and then transfer it to
NES Screen Tool 2.51, adapting it to the right restrictions to make it work in the game...

That way I export the data in
ASM, and format them in
Notepad ++, to finally paste them in the right place in the
rom 
