1. Tools & Settings -> BILLING -> Billing Summary
2. Click on blue button "Make a Payment"
3. Add card details (keep the box unchecked for recurring payments)
4. Type in your desired amount and click on "Make a Payment"
You just need to do this periodically depending on how quickly you're spending on Adwords. We advise logging in and making a payment every few days (and waiting longer if you want the balance to build up before you pay down the bill).