It's hard to find an unbiased link via Google, but there are some research documents that might be helpful. Selection guidelines are not only party specific, but the process is specific to each state.
Here's an interesting historical look at the Democratic Party process, from 2008. (
"A History of 'Super-Delegates' in the Democratic Party")
The note says 'The following is adapted from "Structure as Strategy: Presidential Nominating Politics Since Reform," a doctoral dissertation submitted to the political science department of the University of California, Berkeley in 1986.'
