Since you apparently haven't decided on a specific MCU yet, I'd recommend you proceed to do that.

Programming information for the classic i8751 is included in the original datasheet. I doubt you'll want to use that part, however.

There are many variants of the original, today, and you must choose one. There are quite a few that are easy to program. There are also numerous programmer boards already available at quite reasonable prices, so it's not necessary that you build your own, though that's not ruled out in any way. Buying the components and tools necessary to build one on your own will undoubtedly cost more than buying one, and the likelihood of success with a ready-built programmer is nearly 100%. The chance that you'll succeed with your home-built version is not so certain.

Just remember that there are many ways to "skin a cat," and that you have to make the necessary decisions before you start.

Maxim/Dallas' DS89C4x0 series of "Ultra-High-Speed" microcontrollers are extremely easy to program via the serial port, requiring only three external components including an oscillator, a MAX232 or equivalent, and a 74HC125.

Some other manufacturers have similarly simple programming circuits, though that certainly doesn't imply that they all do. If you do a little reading of datasheets and user manuals, you'll quickly learn which ones these are.

RE