Did you not want to read the paper? It's really short and clearly written.

They just randomly assign people to group A or B. Group A is told about the actual theory that willpower is a finite resource (“Working on a strenuous mental task can make you feel tired such that you need a break before accomplishing a new task”). Group B is told a competing theory that willpower compounds (“Sometimes, working on a strenuous mental task can make you feel energized for further challenging activities”). They administered surveys to confirm that they had successfully influence each groups opinions.

Then, the experimenters tested the degree to which using willpower became harder or easier throughout a task. On average, group A exhibited willpower fatigue but group B did not.