In October 1947, Stalin founded the Communist Information Bureau (Cominform) to control the governments of eastern Europe, this organisation allowed Stalin to find leaders who were against him and replace them with loyal leaders:

How each country in Eastern Europe was taken over:

East Germany - The Allies had given the USSR control here
Poland - After the war, the communists joined in coalition governments, becoming outright leaders when they forced the non-communist leader into exile in 1947
Czechoslovakia - When the Communist party position in coalition was threatened in 1948, they banned other parties. Czechoslovakia became a one-party, communist state
Hungary - Communist party leaders became the largest party in 1947, banning opposition and attacking church leaders
Romania - In 1947, the monarchy was banned and communists took over fully
Yugoslavia - Marshal Tito had Yugoslavia expelled from Cominform in 1948
Bulgaria - A left-wing coalition won elections in 1945, communists members then executed leaders of other parties
Albania - Communists gained power directly after the war, backed by Yugoslavia and USSR