The hardest part of researching CIA operations has to do with the huge holes between declassified documents which after spending over a year researching the early period I know all to well. You should first look to see if the CIA declassified a document that was written by a CIA historian. These are written to cover the history of their own operations and are often used within the agency, however they are occasionally declassified but still remain heavily redacted. I would recommend the national security archive website which is run by George Washington University which has a nice selection of documents that are curated for research. Even if they don't have a ton of information on the particular event your looking for it will still give you a good idea on how to assemble documents for writing. If you have the time I would glance through the info on operation Ajax in Iran which took place during the very early days of the CIA because the related documents are laid out very nicely to put the bigger picture together. Also aside from the papers written by CIA Historians for CIA use there are no real definitive papers that say when something starts, it is more of a series of papers that need to be put together to get the bigger picture.