You can download the programming specs from the MicroChip website at **broken link removed**. Bear in mind the programming for the 876 and 876A are different (EEPROM as opposed to FLASH).
There are already loads of programmers out there, most based on the original David Taid design for the 16C84 - do you have a particular reason to make your own?.
If you look at programmers like the P16PRO40 - a typical example of a 'David Tait' programmer - the only 'difficult' part is likely to be the buffer IC. You can simply replace each section of that with an NPN transistor and two resistors, one feeding the base, and the other from base to emitter (you could probably also get away without the second resistor).