Short answers, pulled from the story.
Harry S. Truman, Winston Churchill, and Chiang Kai-shek signed the Potsdam Declaration on the 26th of July 1945. Truman wrote the signatures for both the Generalissimo and the Prime Minister himself after receiving their approval.
The Soviet Union agreed to adhere to the Potsdam Declaration on the 8th of August 1945. Stalin committed to subscribing to the declaration upon entry into the conflict which was scheduled for early August.
Japan faced prompt and utter destruction if it did not surrender according to the Potsdam Declaration. The document warned that failure to accept terms could expect a rain of ruin from the air never seen before.
Emperor Hirohito announced acceptance of the Potsdam Declaration at 1200 JST on the 15th of August 1945. This announcement followed the atomic bombings of Hiroshima on the 6th of August 1945 and Nagasaki on the 9th of August 1945.
Japanese sovereignty would be limited to Honshu, Hokkaido, Kyushu, Shikoku, and minor islands determined by the victors. The text specified these four main islands plus minor islands as the boundaries for post-war Japan.