Log in to your BitPay merchant account and click on "Donation Page" under "Payment Tools":
Before proceeding, make sure to read the disclaimer and check the box affirming you've read the disclaimer. If you do not see the disclaimer, click on the words 'this disclaimer'. The disclaimer follows for reference:
Once you've entered in the required fields, uploaded your profile image, and chosen the preferred customization options, be sure to click "Save Changes" to save your chosen preferences.