Custom donation amount

I’m afraid I don’t quite understand what you’re trying to accomplish. Do you mean the fields with the suggested donation amounts?
To activate this style, select the “Radio/button selection” option under “Layout of Amount suggestions” on the “Donation Form” tab of the product editing page. Then enter the suggested donation amounts under “Amount Suggestions”.
Docs: https://wcdp.jonh.eu/documentation/getting-started/donation-amount-styles/#RadioButton_selection