Short answers, pulled from the story.
The Soviet Union was formally dissolved on the 26th of December 1991 by Declaration No. 142-N of the Soviet of the Republics of the Supreme Soviet. Gorbachev had resigned the previous day, the 25th of December, and transferred his presidential powers, including nuclear launch codes, to Boris Yeltsin.
The dissolution resulted from several overlapping factors: chronic economic stagnation, the unsustainable financial burden of the arms race with the United States, intense ethnic nationalism and separatism within the republics, and the destabilizing effects of Gorbachev's reforms, particularly glasnost and perestroika. These forces interacted and reinforced each other through the late 1980s and into 1991.
Lithuania was the first Soviet republic to declare full independence, on the 11th of March 1990, when its newly elected parliament proclaimed the Act of the Re-Establishment of the State of Lithuania. Estonia had earlier declared state sovereignty within the Union on the 16th of November 1988, but that was not a declaration of full independence.
The Belovezha Accords were signed on the 8th of December 1991 by President Boris Yeltsin of Russia, President Leonid Kravchuk of Ukraine, and Chairman Stanislav Shushkevich of Belarus. The agreement declared that the Soviet Union no longer existed and established the Commonwealth of Independent States to replace it.
The Baltic Way was a peaceful demonstration on the 23rd of August 1989 in which an estimated two million people formed a human chain stretching 600 km across Estonia, Latvia, and Lithuania. The event marked the 50th anniversary of the Molotov-Ribbentrop Pact, which had led to the Soviet occupation of the Baltic states in 1940.
On the 19th of August 1991, Communist hardliners and senior military figures attempted to overthrow Gorbachev while he was in Crimea. Boris Yeltsin publicly resisted from the Russian parliament building. The coup collapsed within three days. Its failure accelerated the independence declarations of Soviet republics, as the central Moscow government lost its remaining authority.