It’s really easy. All he has to do is get 3/4 of the State legislatures (38 of 50) to pass it...............
If you carefully read Article 2, it merely says “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 Congress;...”
By tradition all states allow every candidate, including minor party candidates, to name a slate of Electors. When we vote for a candidate, we are actually voting for the slate of electors “pledged” to that candidate. The slate that gets the most votes elects that candidate for that particular state.
There is nothing in the Constitution to prevent all kinds of monkeying “in such manner as the legislature thereof may direct...” If you don’t think leftists can manipulate state legislatures to alter the mechanics of presidential elections to their own benefit, you don’t have enough fearful respect for them. They can and will try to do it.