First you have to find the organization. Go to the "Explore Causes" tab and search. Results will appear below your search parameters.
If you're not finding the right results, it could be an issue with how the organization's name is formatted – for instance, if they use "Org_Name NYC" instead of "Org_Name New York." If the charity is US-based and you know its US EIN (tax ID), you can search by that as well.
Select the result you want. You'll see a page like this:
Click the "Donate" button in the "Donate via Wallet" section and you're done!
If you want to donate with your own credit card or ApplePay, click the "Donate via Credit Card" button and you'll see a form that collects necessary information.