After spending some hours in getting my icon to work properly, I finally reached the Step 2.
Instead of following the tutorial's method which uses Hex in order to modify the Type Effectiveness table though, I decided to use esperance's Type Effectiveness Editor 3 which apparently worked just fine judging how 24F050 has nothing but FFs until 24F21D. It looks like the tool deleted the old table and automagically told the game to use the new one.
Anyways, the problem I'm having right now which I don't think is related to the tool though, is that I can't modify an attack's type in order to test if the stuff's working correctly.
I added the line 17=Fairy under the line 11=Dark inside the [Types] section of G3T's \Customisation\Moves Editor.ini file.
Then I changed Squirtle's type to Dragon just for the test's sake and when I was about to change Scratch's type to Fairy, G3T gave me this error:
Halp?
EDIT: Oh yeah, the 1st picture's error which is in Spanish, means:
"You're referring to an item that isn't established as an item" or something like that.
It's as if the program wasn't recognizing my Fairy type.