A Jackpot can be won by getting Jackpot symbols on all reels. The paytable in the lower left of each machine will specify how many symbols it needs. 

The Jackpot amounts are displayed in the upper right corner of the screen. If it is covered by an "x", you will need to increase your bet to qualify for a Jackpot.  The type of Jackpot (regular, Mega, or Epic) you win is determined by the various Jackpot symbols.

WILD symbols do not count toward the Jackpot.

Other than that, it just takes a bit of luck!