Good news! You find yourself on the golden path of Mocktail usage. Creating mocks of instances of classes that you or your team have authored and can readily change is right in the crosshairs of what the library was made to do! (If you're just starting out you should probably target >90% of your Mocktail usage to be of this variety.)
Exactly how you create these mocked instances depends on how you prefer to get dependencies into the hands of your subject under test.
Manually pass instances of dependencies to your test subject, AKA dependency injection.
Allow your subject to instantiate its dependencies by wielding ✨mocking magic✨, AKA dependency inception.