This process takes place every four years in November. The winning electors then cast their ballots in the electoral college in December.
The number of electors in each state is equivalent to the number of representatives the state has in the House of Representatives plus the number of senators each state has in the upper house, the Senate.
The number of representatives is determined by population. But every state has two senators.
As it stands now, there are 538 electors: the equivalent of 435 representatives, 100 senators, plus three electors from the nation’s capital, Washington DC (which always has the same number of electors as the least populous state).
A candidate needs an outright majority of 270 in order to win.