OSDev does indeed have some great resources, but sadly most of the info is targeting x86. Of course the general principles of interrupts, framebuffer memory etc. can be applied to other platforms provided you have an understanding of the platform. Which I guess is the problem - there are tons of great resources, however to fill in the gaps between them requires a knowledge of the platform which a beginner won't have.

The best approach I've seen so far is the excellent Nand2Tetris, however it can't run on hardware due to the custom HDL it uses.