This series of videos is available here: https://eater.net/6502
These first four explore the basic 6502 and get a primitive "Hello World" program running.
https://www.youtube.com/watch?v=LnzuMJLZRdU
https://www.youtube.com/watch?v=yl8vPW5hydQ
https://www.youtube.com/watch?v=oO8_2JJV0B4
https://www.youtube.com/watch?v=FY3zTUaykVo