Here's some info from wiki:

Original plan
Article II, Section 1, Clause 2 of the Constitution states:

Each State shall appoint, in such Manner as the Legislature thereof may direct, a Number of Electors, equal to the whole Number of Senators and Representatives to which the State may be entitled in the Congress: but no Senator or Representative, or Person holding an Office of Trust or Profit under the United States, shall be appointed an Elector.

Article II, Section 1, Clause 4 of the Constitution states:

The Congress may determine the Time of chusing the Electors, and the Day on which they shall give their Votes; which Day shall be the same throughout the United States.

Article II, Section 1, Clause 3 of the Constitution provided the original plan by which the electors chose the president and vice president. Under the original plan, the candidate who received a majority of votes from the electors would become president; the candidate receiving the second most votes would become vice president.

The original plan of the Electoral College was based upon several assumptions and anticipations of the Framers of the Constitution:

Individual electors would be elected by citizens on a district-by-district basis.
Each presidential elector would exercise independent judgment when voting.
Candidates would not pair together on the same ticket with assumed placements toward each office of president and vice president.
The system as designed would rarely produce a winner, thus sending the election to Congress.
According to the text of Article II, however, each state government was free to have its own plan for selecting its electors, and the Constitution does not explicitly require states to popularly elect their electors. Several different methods for selecting electors are described at length below.

Breakdown and revision

The emergence of political parties and nationally coordinated election campaigns soon complicated matters in the elections of 1796 and 1800. In 1796, Federalist Party candidate John Adams won the presidential election. Finishing in second place was Democratic-Republican Party candidate Thomas Jefferson, the Federalists' opponent, who became the vice president. This resulted in the President and Vice President not being of the same political party. . .

Responding to the problems from those elections, the Congress proposed the Twelfth Amendment in 1803 – prescribing electors cast separate ballots for president and vice president – to replace the system outlined in Article II, Section 1, Clause 3. By June 1804, the states had ratified the amendment in time for the 1804 election.

Evolution to the general ticket

Alexander Hamilton described the framers' view of how electors would be chosen, "A small number of persons, selected by their fellow-citizens from the general mass, will be most likely to possess the information and discernment requisite to such complicated ." The founders assumed this would take place district by district. That plan was carried out by many states until the 1880s. For example, in Massachusetts in 1820, the rule stated "the people shall vote by ballot, on which shall be designated who is voted for as an Elector for the district." In other words, the people did not place the name of a candidate for a president on the ballot, instead they voted for their local elector, whom they trusted later to cast a responsible vote for president. . .

Meetings

The Electoral College never actually meets as one body. Electors meet in their respective state capitals (electors for the District of Columbia meet within the District) on the Monday after the second Wednesday in December, at which time they cast their electoral votes on separate ballots for president and vice president.

Although procedures in each state vary slightly, the electors generally follow a similar series of steps, and the Congress has constitutional authority to regulate the procedures the states follow. . .

Each state's electors must complete six Certificates of Vote. Each Certificate of Vote must be signed by all of the electors and a Certificate of Ascertainment must be attached to each of the Certificates of Vote. Each Certificate of Vote must include the names of those who received an electoral vote for either the office of president or of vice president. The electors certify the Certificates of Vote and copies of the Certificates are then sent in the following fashion:

One is sent by registered mail to the President of the Senate (who usually is the incumbent Vice President of the United States);
Two are sent by registered mail to the Archivist of the United States;

Two are sent to the state's Secretary of State; and

One is sent to the chief judge of the United States district court where those electors met.

A staff member of the President of the Senate collects the Certificates of Vote as they arrive and prepares them for the joint session of the Congress. The Certificates are arranged – unopened – in alphabetical order and placed in two special mahogany boxes. Alabama through Missouri (including the District of Columbia) are placed in one box and Montana through Wyoming are placed in the other box. Before 1950, the Secretary of State's office oversaw the certifications, but since then the Office of Federal Register in the Archivist's office reviews them to make sure the documents sent to the archive and Congress match and that all formalities have been followed, sometimes requiring states to correct the documents.

Faithlessness >>>

https://en.wikipedia.org/wiki/Electoral_College_(United_States)