Since you asked for the actual procedures...got them and now want to respond with someone's opinion in a Politifact article from 2017
I'll respond with a more current Politifact article that says you're wrong and actually incorporates Aftergood comments from your article
https://www.politifact.com/article/2022/aug/11/could-trump-argue-declassified-documents/
Could Trump argue he declassified the documents found in the Mar-a-Lago search?
IF YOUR TIME IS SHORT
• The search warrant served on former President Donald Trump has raised some novel legal questions involving national-security classification.
• If the federal government pursues legal action against Trump over document laws, one obvious defense for Trump might be that he declassified these documents before leaving office.
•
Sitting presidents do have wide powers to declassify documents, but they are supposed to go through a detailed procedure to do so.
• This case is so unprecedented that experts urge caution before projecting how courts may respond to such an argument.
Here's the relevant language for you...You're welcome
Could Trump argue that he declassified certain documents in private, while president? That is not how the system is designed to work.
"Merely proclaiming a document or group of documents declassified and doing nothing more would not suffice," Bradley Moss, a Washington, D.C.-based lawyer who works on national security cases, told PolitiFact.
Follow-through is required.
"He had to identify the specific documents he was declassifying, he needed to memorialize the order in writing for bureaucratic and historical purposes, and he needed to have staff physically modify the classification markings on the documents themselves," Moss said.
"Until that was done, the documents, per the security classification procedures, still have to be handled, transmitted and stored as if they were classified."
Tom Blanton, director of the National Security Archive at George Washington University, agreed.
"If the documents are still marked classified 18 months after their removal from the White House," Blanton told PolitiFact, "then Trump was too busy to order them declassified at the time."