It's almost exactly the same, except the building blocks are much simpler (=more primitive) so you have to spend more time per useful thing you want to get the machine to do.

If you want to get a taste of what that means that is definitely 100% within your abilities, try to write a Brainfuck interpreter in JS, and a Brainfuck to JavaScript compiler (that yields a string that you can eval() to run the program).

Then try to write some simple Brainfuck programs, then try to write a Brainfuck interpreter in Brainfuck. At this point you won't yet know how to write an operating system, but you'll definitely know how it feels to write one.