There is no safe method. That said you can try betting a small amount but you will lose in the end. Its the casino owners who make a profit and the players are the losers.
If gambling was a safe method to earn money then people would not study hard and get jobs to earn money.
Yes, Gambling should not be considered as a source of income cause It doesn't guarantee that you will earn from it. Just gamble for fun and only use your extra money