How to create mocks for unit testing in an extensively created existing app - Xamarin

I'm pretty new to unit testing. I recently joined a new company, which builds mobile apps, where I was given the task to create mocks for the different services like locations, internet and web services that used in each platform (This is for an application that is already deployable). Most of the tutorials and forums indicate that creating mocks should be at the start of a project.

How do I get around this and build mocks for unit test?

Thanks in advance for any help and advice.

Cheers!