Just leave the box blank that says 'Your name (optional)'. This is in the first step of the donation process. To hide your donation amount from the page, make sure to tick the box next to 'Hide my donation amount from others'.
If you're donating to a charity, although you'll appear as 'Anonymous' on the Fundraising Page, the Page Owner will know who you are and how much you've given by logging into their account.
When donating anonymously to a Crowdfunding Page, the Page Owner will not be told who you are.