For this one in particular and many of my other songs I've done, I noodle around in my editor with differing notes and instruments, until I hear a melody/flow in my head, which is usually all it takes to start a full song. Initially I didn't really have a clear theme direction to go in, until I internalized the text theme, and made it more traditionally "Doomy". For each of my songs, I really try to make them all individually memorable in their own ways, usually I do this by employing techniques I haven't before, and several of them to get a very diverse sound throughout each song I do.