At the end of the Kickstarter you will be sent a Kickstarter Survey. Use the survey to tell us how you want to spend your pledged funds.
For example, if you ordered the Kaiju Junior tier for $25 but you would also like to add-on a Kaiju "Cookie Monster" plushie, go to Manage My Pledge and add $25 more to your Pledge Amount (for a total of $50). You will go through the payments process again, but don't worry, your card will not be charged multiple times. The new pledge will overwrite the old one. After all that, when the Kickstarter closes, watch for a survey in your email.
International backers should pay special attention to the shipping fees and adjust their pledges accordingly.