How to create test case for a function currently under development?

You can create some manual test cases after the study of the function to be tested.
And if the script is to be written for the function testing, you can write some raw script for that and check the functionality.

We can test a function with the help of documents given like Business Requirement Document or Functional System Design Document because they contain the info regarding functionality of the application, on the basis of that we can create scenarios and functional testcases.

As known to me, you cant test a piece which is under development. Only when the developer completes the implementation and done with the unit testing, it should be delivered to the test team. If at all you are forced by the management to test an item under development, it will be a double work for you since you need to test the same once the entire piece is fully developed.

You can refer to requirement document such as "Business Requirement Specification" document, "Technical Specification" Document, User Story, Use Case document to get the inputs for test case. You further communicate with BAs and developemnt team to understand the functionality for writing test cases. Once the function is clear to you , you can make multiple test scenarios on the basis of different buisness path.

A function can be tested if application is in developing state
if it is a web service call we can do web service testing
else unit testing of the function can be done to make sure that the function is working fine as per the requirement.