Firstly, get the gentleman's attire by signing up for the Rockstar Social Club website and linking it to your Xbox Live or PSN account. Then when you've done that you can use the Gentleman's attire to enter the high stakes poker game at Blackwater, which is the poker game that shows up on the map that is NOT at the same place as your save point.
Once you're playing poker the best advice is to just be patient. Don't try to bluff the other players as it is likely that they will just call you and you'll lose, so wait until you have a really good hand such as a flush or a straight and then just bet enough to raise the pot over 2000 - that way you will earn the achievement even if everybody else folds. Also by not going all-in you will still have some chips left to try again in case you are called and someone else has a better hand than you.
Go to the Landon Ricketts mission 'Lucky in Love' then bet all your chips in 1 hand (if you get 1015 chips carry on), then bet 1000 chips in the next round, if you win then you will get the cheevo/trophy.