You have to describe the integration of two systems (at least two)
and describe what data is exchanged between applications, eg the integration of the application for the purchase of parking tickets with the applique of the controller that checks these tickets, but this is just an example.
the best way to do it is to integrate the payment process with the application, additionally you have to write out what error codes can appear, how to protect yourself against SQL injection
and in addition, some block diagrams, screenshots about what it would look like
and decide whether it is the rest API or the latter, justify why.
I made this projects in points in pdf here : [login to view URL]
Please let me know what kind of aplications you want to connect