I mentioned in a previous post that Ben Eater has a brilliant series of videos on building a computer from scratch.
He had a new series which is very good on building a computer using a 6502 microprocessor.
This is the microprocessor that I know best as it was in my Apple ][ and in my Atari 800.
Here is a link to his website where you can watch the videos (there are eight as I wrote this) and also buy the kit from him.
Here is the first video in the series: