There are two ways to log in to an existing account. If you don't yet have an account, don't worry, you will be given the option to create one after placing an order.
Log in button
At the top right of the page is a button marked "Log in", click this to take you to the login page for existing customers.
You will be asked for your email address and password:
If you can't remember your password, just click the "Forgot your password?" link to go to the reset function.
If the "Remember me on this computer" button is checked, your browser will save your login details. You should make sure this is unchecked if you are logging in from a public computer.
While placing an order
If you have selected your items and go to checkout before logging in, you will be asked to enter your email address, and if you have a password already.
If you have an account already, enter your email address and password under the "Returning Customer" heading and click "Login & Checkout". If you can't remember your password, just click the "Forgot password?" link to go to the reset function.