There may be several differences across the various casinos, but usually you have to obtain casino’s native tokens and stake them to be able to receive a part of the casino profits.

1. Play games on the particular casino to mine their tokens or buy (if it’s possible) tokens on crypto exchanges
2. Stake or “freeze” these tokens 
3. Receive dividends – a share of casino profits (these are be paid out daily, weekly, etc.)

So first you have to invest money into playing on the casino or buy directly token on exchange (not everytime there is this option). Then you will be qualified for earning dividends, casino systems are usually set to distribute share on daily basis, some will send out funds weekly or the use some different time period.