Go to the Simcountry portal page (upper left), click on Cash Market link (left side), you should see a set of links for Population Transfers (lower left), click on "bid coins for population transfer".
You'll then be taken to a page listing blocks of 1 million population each (some that other players are selling and some that the gamemasters are selling). Just click on "Bid" for one (best to choose one someone else hasn't bid on yet). Next you'll be asked which country you want the pop to go to and the number of coins you want to bid. (5 is usually the max most players pay... but some pay more if they really need the pop right that moment, others bid less if they want to try their luck at getting a deal, but the chances are good someone might out-bid you.)