In your dashboard, click on the "Add a New Snapshot" button. You'll now be asked to enter in a page title. This doesn't have to be your page's exact title, just something that will help you remember it.

Next, you'll need to enter in the exact URL of the particular page that you want to test. Select "customize url tracking rules" for more url options.

You'll then be able to set the length of the time and the amount of visits you want to track for. Before you finish, add a description for your test, such as "testing new ad layout."

To make changes in the future, you can always click on "edit this snapshot" from your dashboard.