Actually raw modding in dwarf fortress is quite simple. You're just changing lines in a text editor, and copy/pasting valid lines certainly does work. The
wiki can get you started, there are pages on there where valid tags and tokens are listed and explained, and if you run into problems you can ask over in the modding subforum. (below the adventure mode and community subforums) There are
threads there actually which are basically available for anyone to ask questions at random without having to start their own thread.
I think the things that people get stuck on most often is forgetting to change an object or references' ID and forgetting to add references, like to material and tissue templates, before trying to use those materials or tissues. Usually if something is amiss, the errorlog.text file in your base dwarf fortress game folder (where the .exe is) will mention it.
Edit; to help get started, if you want to make, say, angels, I would start by going to your raw/objects folder and opening the file creature_standard.txt. search for [CREATURE:HUMAN], and copy that line and everything below it until the next [CREATURE:X] token, and paste that into a new text file or the end of the same file (I find organization really helps and so I suggest new file.) Your new file should be named creature_whatever.txt and the top of the file should start with the text "creature_whatever" and then [OBJECT:CREATURE] following that. The line at the top of the file becomes the files name as the game sees it, and object:creature tells the game the following lines use creature tokens and are meant to define a creature. CREATURE:NAME starts the actual definition of a creature, and gives it its identity or ID. That ID is all-important, as everything that references the creature calls on that first and foremost. If the ID of two creature definitions are the same, the game throws a tantrum and does crazy shit like allow you to play as forgotten beasts. This is called duplicate raws. It is hilarious, but can also crash the game.
So pasting your creature:human in there, the first thing you want to do is change CREATURE:HUMAN to something else. Like CREATURE:AKATOS_ANGEL. Now it is its own separate creature and everything you do from then on further defines it.