I suspect the trick to this is maintaining consistency: always doing some time on it every day, and always logging it down. Apparently I had finished Tutorial #4: Buffers, Shaders, and GLSL, but had not yet gone on to Tutorial 5: Texturing.
That makes a certain amount of sense, as, around the time I was doing this last, Rastertek took its tutorials down, and while the Internet Archive had the text, it lacked the textures to be applied.