What's the required procedure for a President to declassify anything? Which law or statute lists the requirement and who checks to see if the procedure was followed? Would it be Trumps boss in the executive branch who doesn't exist? Did Trump have to issue an executive order or list his intent for three consecutive weeks in the legal notices of the Wash Post? The fact he removed the files while still in office declassified them. See BJ Clinton sock drawer tapes.
The most egregious thing here is the willful selective prosecution of Trump to the exclusion of almost everyone else doing the same things who weren't EVEN President! They never even had the privileges of the Chief Executive.
There is none.
As is often the case with these stupid Federal laws, Congress went out and passed criminal statutes related to "classified materials," but never specified what exactly constitutes "classified materials." Instead, Congress left it up to the Executive Branch to establish the protocols for classifying and declassifying sensitive information that is vital to our national security.
So the protocols for classification and declassification aren't established by law. They are established by executive orders and internal Executive Branch rules and regulations -- none of which apply to the President of the United States.