When you want to use your Rewards, you will notice a box under the big pink checkout bar with your Rewards balance (on the shopping cart page). You can choose to use all of the Rewards or some of them (or none of them if you want to keep on saving). Type in the amount you want to use in the box under the balance and then click apply. That amount will be deducted from your
total.
Remember, if you are trying to qualify for the Product of the Month, the product total after the Rewards has been deducted, must be at least $75.