There aren't many guidelines for a convention. States that have referendums, like California, would likely use a referendum to pick delegates.
There was a convention of states hosted by the Arizona legislature in September 2017 that specifically laid down the ground rules at least for that instance. It was the first time anything of that sort had even been held since the 1861 pre-Civil War convention that attempted to stop the war from occurring. There's video records of proceedings at the Arizona legislature's website, and you can go to bbaplanningconvention.org as well.
