You must first get your Champion to the highest Level its current Rank allows. You then sacrifice Champions of the same Rank as the chosen Champion.
The amount of Champions you sacrifice must be equal to the current Rank of your Champion. For example, a Rank 2 Champion must have two Rank 2 Champions sacrificed for it to reach Rank 3.
After the Rank is improved, your Champion’s Level is reset. You must get them up to the highest Level available again to reach the next Rank.