To donate you can visit the donation store.
At the donation store, create an account. This should be a separate name and password from your shard or forum accounts. Protect your information carefully.
This page works like any online store/shopping cart. There are no shipping fees since there are no items shipped. Once you have filled your cart with the items to purchase, you will be able to check out using PayPal.
When the payment completes, and your order status is updated to "Awaiting Fulfilment", you can claim your donation reward by saying the claim command on any of your characters in-game:
The claim form will ask you for the zip code of your entered billing address, the donation amount (in USD) and order number. The order number can be found on your PayPal receipt and in the email sent to you.
If you experience any issues claiming your donation reward, feel free to contact in-game staff by sending in a help request.