These days it is pretty competetive but the basic approach is no different from general marketing.

Start by finding a niche, promote just say 4-5 casinos and also some poker rooms to these niches.

Some niche ideas are:

For casinos 1c or penny slots, high roller bonuses, niches within game types like blackjack that have several variations so target say european blackjack or multi-hand blackjack.

For poker rooms that offer bad beat jackpot bonuses (a bad beat is when your 4 aces gets beaten and these bonuses can exceed 1 million dollars, 50% to the loser, 25% to the winner and the rest divided among the table), less common poker games like tri or three card poker or pan poker.

Write good reviews of the properties you are promoting with original content and you will soon see what is converting for you.