You first need to create an account at a licensed online casino. Then, deposit funds to your account through an accepted payment method. Choose the slots game you want to play, start it up, and set the size of the bet. Now, you just have to press the spin button to play one round.
You just have to stick to legal, regulated online casino sites such as BetMGM, Caesars Palace Online Casino, FanDuel, Betway, bet365 and BetRivers. They offer great casino game ranges and they will pay out quickly and without any issues if you win.