Figure out which candidate was ranked last by the largest number of voters.
That's Coomb's method, and it generally works better than IRV, but has strategy-susceptibility problems.
For every voter who ranked that candidate last, their second-to-last choice is now treated as their last choice.
That's not Coomb's method, but might be equivalent? I'm a little confused by this. Remember that after that candidate is eliminated, they are removed from all ballots, not just the ones that listed them last.