I've tried looking through a few pages of search results in Google, Google Scholar, and JSTOR, and can't find out anything about how they were selected when it was first established in 1976.
Not sure if this is a homework question (hence why I am not spelling out the answer in my comment). The answer is in the Constitution of 1976. They dedicate an entire chapter within the constitution to laying out the powers and electoral process behind the National Assembly of Popular Power.