Log in to your account, and after you activate or manually get Bonus Spins credited to you, open the slot on which your spins are available. Most games will notify you about the number of Bonus Spins available before you start playing.
After you play through your batch of Bonus Spins, any winnings will be added to your bonus balance and will need to be wagered according to the reward’s requirements.
Example: If the wagering requirement is x10, and you have won 10 USD, you will need to wager 100 USD (10 USD winning x 10 wagering requirement).
For more information, check out the Terms & Conditions of the relevant reward.