Before you spawn, obviously make sure you have C4 as your accessory. A tank can take a lot of damage before it goes down, so even if you plant the C4, shoot it up a bit before you detonate.
This is hard to do, considering how overpowered tanks are, especially in the hands of an experienced player. I suggest boosting this using buggies instead. The Base is a nice small map with two buggies (and a tank) to choose from. Shoot the Buggy 18 times with your pistol before blowing it up!
This is cumulative over multiple games/maps, so just keep at it till this pops.