To create a Payment/Donation form, click '+Create New Form'. Select the "Form" option, enter the name of the form, and click 'Create Form'. Edit the basics of the form (including the name, group, introduction and completion message, and privacy settings). Select the "pay/donate" field type and input the title, and amounts, and choose whether you'd like to make it mandatory or optional. When the form is saved, it will be saved to the list of forms. Here's a short gif that demonstrates how to set up a simple donation form:
Please take a look at the following gif that shows how users complete such type of a form:
You can easily track form payments in the Responses tab: