History has shown us two ways to accomplish this.
The most brutal way is submission by military force. It is by far the most horrible because of the suffering it induces on the people besides trying to short-circuit democracy ruling.
The other way is more subtle. Just like the monetary system and the euro is used by the central bank in Europe; In this way one takes over the currency and money supply of a country to make it completely dependent on the authority issuing the money, and then forcing it into submission by demanding them to sell out important domestic economic resources or privatize them by holding back money until resources are sold and political changes are done.
The country under austerity pressure is then taken over.