The Soviets began to move troops eastward in April 1945, following a planning phase that had begun in March 1945. Most of the troops that would take part in Soviet operations arrived in May to June 1945. These troops were some of the most experienced troops in the Red Army, and were chosen for their experience in fighting in the terrain they would be fighting in. For example, the 6th Guards Tank Army, who had been fighting in the Carpathians, were chosen to fight in the Khingan Mountains of Western Manchuria. Similarly experienced generals were chosen to lead this force. Nearly a million men were moved from the European Theatre, under a veil of secrecy that convinced the Japanese that the Soviets wouldn't be able to launch an attack until autumn 1945. There were 1.5 million Soviet troops in the Far East in August 1945. The main Japanese unit in Manchuria and Korea was the Kwantung Army. This had about 1.2 million men, 700,000 of whom were in Manchuria. As was common for the Japanese Army, these troops were well trained, but poorly equipped and supported. They had little artillery support, few anti-tank weapons and obsolete tanks.

The Soviets launched their attack on Manchuria and inner Mongolia on the 9th August 1945. Attacks took place along the entire length of the Soviet-Manchukuo border. In Mongolia Soviet troops made rapid advances, bypassing Japanese fortified positions. By evening on the 9th, Soviet units had begun to secure the city of Hailar, 60km from the border. Soviet troops raced through Mongolia and northern China, making advances of up to 100 km a day. In Manchuria, Japanese resistance was stronger, with strong defence along the line of the Ussuri river. This slowed the Soviets, but could not stop them. Soviet spearheads were making advances of 30 km/day, and would often take casualties from Japanese counterattacks. The Soviets were not helped by poor terrain. However, once they broke through to open country, their spearheads were able to move fast, capturing Harbin on the 20th.

Soviet troops attacked the Japanese on Sakhalin. Pre-war, the island was split roughly in half, with a Soviet north and a Japanese south. Soviet attacks along the border failed to force the Japanese back, but a Soviet amphibious landing on the 16th made these positions untenable. Once the border fell, Soviet troops were able to sweep southwards to Toyohara. Post-war, the majority of the Japanese population was evacuated back to Japan.

The Soviet attack was a massive undertaking, and I've simplified a lot here. For more detail, David Glantz's paper August Storm: The Soviet 1945 Strategic Offensive in Manchuria, found online here, is the classic workk on this topic.