By what means were the Bolsheviks able to take and maintain power in Russia in the period 1917-22?

The Bolsheviks were able to take power in Russia primarily because they were more disciplined and better organized than other groups. They also benefited from their consistent hostility to the First World War, which was becoming deeply unpopular among the Russian people.
After the insurrection of October 1917, the Bolsheviks went into government with members of another left-wing party, the Socialist Revolutionaries or SRs. There was a general consensus among the Left, including some Bolsheviks, that it was necessary for the dual power arrangement of the February Revolution to be kept in place. Under this arrangement, power would continue to be shared between the central government and the soviets, or elected councils representing workers, soldiers, and peasants. The Bolsheviks had managed to secure a majority in the soviets, especially among the workers, and this was their main power base.
According to the teachings of classical Marxism, socialism could only be established in countries where capitalism and all its manifest contradictions had developed to their fullest extent. Then, so the theory held, the conditions would be ripe for socialist revolution. However, Russia was an overwhelmingly agrarian country, with relatively little industry by comparison with the advanced capitalist economies of the West. So orthodox Marxists believed that liberal policies still needed to be pursued for the foreseeable future in order to pave the way for the inevitable socialist revolution.
The Bolshevik seizure of power turned the established orthodoxy on its head. A socialist revolution had taken place despite the relative lack of capitalistic development in Russia. At the same time, the Bolsheviks still committed themselves to going ahead with the election of a new democratically-elected Constituent Assembly. However, when the results came in, the Bolsheviks found themselves in a minority. Under Lenin's orders, the Assembly was dissolved at gunpoint, and the Bolshevik dictatorship began in earnest.
The new regime began to crack down on any sign of dissent. The Bolsheviks' political rivals were gradually banned; civil liberties such as freedom of the press and freedom of assembly were abolished; and a secret police force called the Cheka was established to root out the merest hint of subversion or counter-revolutionary activity. The Cheka—the forerunner of what would become the KGB—quickly gained a fearsome reputation, using the most savage, ruthless methods to suppress the regime's opponents, both real and imagined. Terror was the order of the day, and allowed the Bolsheviks—known after 1920 as the Communist Party of the Soviet Union—to consolidate their hold on power.
The Bolsheviks had gained a lot of support with their radical slogan "All power to the Soviets!" Yet once they had established a one-party dictatorship, they proceeded to grab the soviets' power for themselves, centralizing control. It would be another seventy years before the Communists would relinquish their iron grip on power.