The authoritative source is the 12th amendment:
The person having the greatest Number of votes for President, shall be the President, if such number be a majority of the whole number of Electors appointed; and if no person have such majority, then from the persons having the highest numbers not exceeding three on the list of those voted for as President, the House of Representatives shall choose immediately, by ballot, the President. But in choosing the President, the votes shall be taken by states, the representation from each state having one vote; a quorum for this purpose shall consist of a member or members from two-thirds of the states, and a majority of all the states shall be necessary to a choice. And if the House of Representatives shall not choose a President whenever the right of choice shall devolve upon them, before the fourth day of March next following, then the Vice-President shall act as President, as in the case of the death or other constitutional disability of the President.
Note that the fourth day of March text was changed in a later amendment to the 20th of January.
The state delegations in the House of Representatives vote state-by-state. The candidate with a majority of the states wins. If no candidate wins a majority of the states, it goes to the Vice-President. Note that the Vice-President is chosen (possibly by Senate vote) prior to the presidential selection. And both houses of Congress change prior to the vice-presidential and presidential selections.
The Republicans currently have a clear majority of the state delegations. This is likely to continue. They have a stronger control over the state delegations than they do over the House majority. This is because the small states that Republicans dominate count for as many votes (1) as the larger states where Democrats are most likely to gain seats.
That said, Utah is unlikely to keep presidential candidates from 270. It's a clear Republican state in most elections and only has six electoral college votes. Donald Trump would need to make a major comeback to limit Hillary Clinton to 269 or fewer electoral college votes. If he did that, he'd presumably gain votes back from Evan McMullin. It would be more effective for a Democratic state like Maine, Massachusetts, Minnesota, or New Mexico to go for a third-party candidate, as that would reduce Clinton's overwhelming lead.
It's not clear that a McMullin win in Utah would change anything even if it did keep the candidates from winning the electoral college. He might have trouble rallying Democrats behind him to join with Never Trump Republicans to flip state delegations. It's possible, but not decisive. By contrast, Johnson would have a much easier time appealing to Democrats and still has Never Trump appeal to Republicans. And of course, Johnson has actual governing experience, where McMullin is a first-time candidate.
The more likely effect would be to send the election to the House and Trump voters to apply enough pressure to give him the election. But if Utah voted for Trump, he'd win the election anyway. The greater risk seems to be that a four-way split would give Clinton victory in Utah. But that seems unlikely to occur if Trump is anywhere close to 270 electoral votes.