It seems to me like in the original design of the electoral college, there wasn't any role for the popular vote: the state legislatures would pick the electors, and the electors would have a conclave of sorts where they would debate and vote, without having been pledged to any candidate.

Looking at the Wikipedia article on the 1788 presidential election, it seems like even then some electors were chosen by the people of their states, but it seems like the people were voting for electors, not candidates.

In a modern presidential election, you don't vote for electors, you vote for candidates. For example, if you were a voter in Alaska in 2012, you saw on your ballot the names "Barack Obama" and "Mitt Romney". But if you voted for Romney, you were actually voting for Kristie Babcock, Kathleen Miller, and Christopher Nelson (the three electors from Alaska that year; each had pledged to vote for Romney). In a modern presidential election, the voters vote for electors who have been pledged to candidates without actually seeing the names of the electors.

When did electors start being pledged to candidates? How did the transformation occur?