How do I unit test a function

I have returned to dev after many years. Given the many years, and the changes in c++ dev since I last worked in the field, I consider myself fairly new to dev. I was given the task of learning how to unit test our gui functions using QTest. I read, copied and ran the QString example in qtest tutorial. I have read through the documentation as well. I have not found how to put it all together to actually test a function or class written outside of a qtest file. For example, I created a simple test gui that writes a string after a pushbutton click. However, I am not able to get my unit test to use the gui I created. Can you direct me to a more in-depth tutorial or document that gives a complete overview on how write and implement a unit test?