In terms of practical advice, Aaron has it covered. (As a fundraiser myself, please do let the charity know if you plan to bundle several years’ giving into one).
But I have a more fundamental question: why don’t you want to just pay your taxes?
Taxes are a lot less efficient at doing good than effective charity.
Scott Alexander talks about this, particularly in points 4 and 5, here: https://slatestarcodex.com/2019/07/29/against-against-billionaire-philanthropy/