To pledge to a project, just click the green “Back This Project” button on any project page. You will be asked to enter your pledge amount and select a reward tier. For US-based projects, you will go through the Amazon checkout process. For projects launched outside of the US, you will check out through Kickstarter.
If the project you’re backing is successfully funded, your card will be charged when the project reaches its funding deadline. If the project does not reach its funding goal, your card is never charged.
If you've backed a project that is still live and are seeing a charge on your bank statement, what you are likely seeing is a temporary authorization of funds. The time it takes for an authorization to clear varies on the bank, but typically takes 1 - 3 days. You will not be charged until the project you backed has ended successfully.
Visit the project page and click “Manage Your Pledge.” Enter a new amount in the pledge amount box. Note that you are not adding to your existing pledge; the amount you enter will be the total amount collected if the project is successfully funded.