What does Double Up mean?
Solution 1:
Doubling up is winning a hand, doubling the amount of chips in your possession in the process.
Basically you must go all in and win. Technically to double up precisely you have to do that against one opponent with more chips than you do, but I think winning more than that by going in a showdown with three or more people also counts.
Solution 2:
You don't need to go all in. You can double up by having every player to bet at least 1/4 of what you have.