This is quite a long story, but what I think you want is the final days summary. To get there, first we need to do a fast forward through the 1980s so we can reasonably sum up events to December 1991.

1985 is about when the collapse started. Mikhail Gorbachev grew up in the realm of agricultural economy and came into the Politburo looking to improve that economy. In 1985 he was chosen to become the General Secretary of the CCCP. To fulfill his goal of economic improvement he recognized that he would have to drastically reform some Soviet policies. This is where glasnost and perestroika come in. Glasnost means "openness" and perestroika means "restructuring". These would be his major reforms.

Throughout Gorbachev's time as General Secretary the Soviet Union began reducing its grip on everyday life, economy, and political expression. By 1989 Latvia, Estonia, Ukraine, Belarus, and Lithuania all had growing resistance movements. Azerbaijan, Georgia, and Armenia had violent resistance movements appear. Gorbachev abandoned the doctrine of interfering in all Warsaw Pact nations' affairs.

This is when Gorbachev led a limited Democratization initiative. With many of the member states of the USSR facing a popular uprising of political expression, protests arose everywhere. It wasn't until the fall of 1989 that the Soviet Union responded militarily in Georgia.

In 1990 Gorbachev changed his position from Chairman of the CCCP/General Secretary to President of the Soviet Union. He recommended the Communist Party of the Soviet Union relinquish its position as a single party for the Union, and as a result the Communist Party lost elections in Lithuania, Moldova, Estonia, Latvia, Armenia, and Georgia in 1990. It was in March 1990 that Lithuania became the first full member of the USSR to declare independence.

Also in 1990 was when Russia began usurping the Soviet Union's position. With the Soviet Union's sudden caving to public wishes, many Russian Federalists saw an opportunity for Russia to gain power. The head of the Russian Federation was Boris Yeltsin and he resigned from the Communist Party on 12 July 1990.

We are now caught up to 1991.

At this point the Soviet Union has lost control of the Warsaw Pact's members and their satellite states, the Communist Party of the Soviet Union has lost control of six member Republics, Lithuania has declared independence, and the head of the Russian government resigned from the Communist Party.

In the first half of 1991, Soviet forces attempted to suppress the independence movement in Lithuania, but largely failed and strengthened the resolve. A referendum was held determine public support for a continued Soviet Union, but Armenia, Georgia, and Maldova all boycotted the referendum resulting in a 76% result favoring a continued but "reformed" Soviet Union. Boris Yeltsin was elected to the new position of President of Russia. Yeltsin campaigned for the post criticizing what he called the "dictatorship of the center".

By August 1991 the Soviet Union had lost control over almost all of its political arsenal. In response, Gennady Yanayev led an attempted coup in August to regain control over the Soviet Union, undo Gorbachev's reforms, and prevent the collapse. This was the hardline Communist Party's last effort to save the Union. Yeltsin led a resistance from the Soviet White House with citizens of Moscow assisting in building barricades around the building. The barricades held and with the inability to take the 'White House', the military coup was withdrawn. By the end of the third day of the attempted coup, Gorbachev was back clearly in power.

This was the final major blow to the Soviet Union's existence. Three days after the end of the coup Gorbachev resigned from the Communist Party, his replacement resigning on his own five days later. In between the two resignations Moldova declared independence. Then the Supreme Soviet suspended all Communist Party activity in Soviet Union territory. The Supreme Soviet replaced the "Congress of People's Deputies of the Soviet Union".

In September Estonia, Latvia, Tajikistan, Armenia, Turkmenistan, and Ukraine were all recognized as independent. In November, only Russia, Belarus, Kazakhstan, and Uzbekistan had not become independent. In December final agreements were made to make the Russian Federation the successor state to the Soviet Union as the Soviet Union had succeeded the Russian Empire. On 24 December the United Nations was officially informed of the intent, on 25 December Gorbachev resigned and the Soviet flag was lowered at the Kremlin for the last time, and on 26 December the Soviet Union officially ended.

https://history.state.gov/milestones/1989-1992/collapse-soviet-union