If Ron Paul became POTUS and wanted to begin undoing the Military Industrial Complex by first ending the officially declared "War On Terror", what would he need to do to officially do that?
There is no constitutionally or statutorily prescribed means of ending a declared war. One possibility is that the parties stop fighting because there was no reason to continue, and they sign a treaty (English and the War of 1812). Another is that one side stops fighting and surrenders to someone else (Rumania surrendered to the USSR though the US had declared war). Germany surrendered on May 7 1945, but hostilities with the US were not officially over until December 13, 1946 when Proclamation 2714 was signed by Truman. The core of the proclamation was
Now, Therefore, I, Harry S. Truman, President of the United States of America, do hereby proclaim the cessation of hostilities of World War II, effective twelve o'clock noon, December 31, 1946
but he didn't even say who the other guys were.