Most bare-bones way to use a CPU? (How to learn)
Sounds like you want to work with a microcontroller. You can write assembly language code easily for an 8-bit microcontroller. You could use an Arduino board and AVR Studio and program it through the ICSP header with an appropriate tool. Or a PIC etc.
It's feasible to program ARM and even more complex chips in assembler but there is a lot of tedious setup and it would not be pleasant (often we use tools just to crank out the configuration code with more complex chips).
With a simple 8-bit processor there might only be 5 or 10 arcane instructions and some configuration variables set up to get it to chooch.