I would say bit by bit, I mean "small part" and then keep making small extensions to the program(s)

First for new hardware would be a monitor program. Either write one, or port one from other hardware. A monitor program allows the inspection and edit of memory data and also to run programs in such memory. The monitor program often placed in ROM for convenance. A 6502 cross assembler, could be used to avoid the need for hand assembly the program to 6502 machine code.

Probably where WOZMON came from.

Then using WOZMON start writing a VERY simple basic interpreter.