How to add new custom fields in existing GraphQL API (Query and Mutuation)?

I have basic knowledge about GraphQL. Now, I would like to convert my knowledge into action. For that, I want to know about few things like how can I add my custom field to any GraphQL Query and mutations. I have also read some blog posts for this but they don’t have proper guidance which I am looking for.

Can someone please explain with example of extending an existing GraphQL API for both Query and Mutations? That would be really handy.

Thanks in advance!