Automatically. When you book something a credit covers and the booking meets the minimum spend, the credit comes off. There's nothing to select and no code to enter.
You can't save a credit for later. If a booking qualifies, the credit is used. This is deliberate — credit expires at the end of its period, so holding it back is the easiest way to lose it.
You'll still need the full amount in your account when you pay. Rewards credit is applied to the transaction, not to the payment going through, so make sure you have enough to cover the booking before you check out.
If a booking is below the minimum spend, no credit is used and it stays in your account for next time.
Occasionally a payment ends up clearing for less than you were charged at the time — a cancelled extra, or an adjustment by the hotel. If that takes it below the minimum spend, we return the credit to your account so you can use it somewhere else within its period.