Looks like your mirage setup looks good. If you’re developing/prototyping you’ll need to create some data in your mirage scenario (just a donor, and then the 3 payments will be created automatically) and then load them in your app.
As far as how to access them once the donor record is loaded in your app (presumably a findRecord('donor', donorId) in a route model hook) just reference the relationship via donor.get('pay') or, in a template:
{{#each donor.pay as |payment|}}
{{payment.donation}}
{{/each}}