Play a Single Player, Head to Head, No-Limit, Ring Game and go ALL-IN every hand. The AI will fold more often than not and you will rack up 500 XP each hand for a winning streak. This works until level 25
After level 25 you will need to use multiplayer to gain experience. Experience is earned for playing hands, winning hands, and winning/folding in certain ways. The easiest way to earn this is to find a boosting partner and switch off winning for a number of hands, similar to how you got to level 25 in single player.
If you have a turbo controller, you could also try to find a group with others with turbo controllers. You will want to set up a table and turbo . This will cause to fold. You will earn 10 XP for each person at the table. You will not earn as much experience with this method, but you can AFK boost this method.
A quick way to earn additional XP is to complete all the Pro-Takedowns for the initial xp bonus. You can also use XP Boosters during regular gameplay to increase XP earned.
You will reach level 50 once your character has accumulated 1 million XP and level 100 at 2820000 XP.