How did the Bolsheviks take over Russia?

The Russian Civil War was not the means of seizing power, but of consolidation.  The Bolsheviks were firmly in power by Christmastime of 1917, and the Civil War raged through 1921.  The Bolsheviks seized power because of three main reasons, the weakness of the Provisional Government, the cohesion of Lenin's followers and the fact that he had ready money and his opponents did not.

The Kerensky government, formed after the May, 1917 overthrow of the tsarist regime, was determined to stay in World War I to support the French.  The people as a whole were sick of the war and consequently the Provisional powers lost the support of the masses.  The government's inability to effectively deal with hunger in the cities or control the tsarist counterrevolution or the left-wing agitators of the more radical socialist, communist and anarchist factions made things worse.  The moderate liberals of Kerensky's government simply lost control.

Lenin's reputation with the more radical factions was immense, although many came to see him as a potential tyrant as the year continued.  The Social Democrats split into the Menshevik and Bolshevik factions ("minority" and "majority" respectively, although the Bolsheviks were in fact a small minority of the party), and none of the other radical parties had any real power base outside of very localized centers, usually in Moscow.  Lenin's ruthless pursuit of one central goal served him well, as did his grasp of propaganda and the competence of his followers (Stalin, Trotsky, etc.).  The third factor was simply having funds at hand, something no other political group had.

When the Germans put Lenin on a sealed train in the spring of 1917 and sent him from Switzerland to Moscow they put on the train with him one million dollars US in gold bullion, which was an amount able to finance a great deal of activity. No other faction in the revolution had such ready cash, and the government was barely able to economically function at all.  These things, combined with the practicality of Lenin's plans (and Trotsky's military acumen) for seizing food in the countryside for distribution in the cities, the military attacking of tsarist forces and other so-called "counterrevolutionary" groups (which meant, in the end, all of Lenin's enemies) and the Czech Legion fighting along the Trans-Siberian railway lines, lifted him to leadership.

This, of course, brings up the question of who financed Lenin.  The answer is that the money came through Swiss banks from a consortium of Swiss, British and American businessmen and bankers.  After the initial funds Lenin carried to Russia with him the Bolsheviks were financed to the tune of at least $20 million by, among others, Jabob Schiff, Sir George Buchanon, Olaf Ashberg of the Nye Bank of Stockholm, Albert Wiggin and W. B. Thompsaons (president and a director respectively of the Chase National Bank of New York), and various directors of Royal Dutch Shell and Standard Oil, among many others.