How to make custom armor textures WITHOUT using a template ?

Is there any particular reason why you don't want to use a template? Because my advice would probably be to just start off with the template, then add, remove, and edit parts of the texture as you see fit until it no longer looks at all like the template.

If you just want to make a blank armor texture, you could use the template, then erase everything.

Alternatively, if you have an external armor texture that you want to use, you could make sure that the file names end in layer_1.png and layer_2.png respectively, then import it into MCreator.