This blog post applies to SharePoint as well as any custom software your write.  This is the high level user acceptance testing that should be done when finishing a every custom project you do.

1. Write User Scenarios/Requirements
2. Develop Code based on requirements
3. Create a test plan, which is all of the tests your will document and run
4. Write test cases based on the requirements and user interface
5. Run test cases to make sure everything is working
6. (optional) Use a subset of tests as a smoke test

Feel free to debate, suggest or recommend any changes to the steps.

Here is what the testing process might look like.

Testing Process

1. Go to the folder location where you have stored the testing documents such as http://<sharepointsite>/<project name>/Testing/UserTests
2. Open a testing document such as 01-UserTests-<component>.docx
3. Record results in a new Test Run document with name [test##]-[YYYYMMDD]-[user]-testresults.xlsx (example: 01-20100610-bschwartz-testresults.docx)
4. Run through every test in testing document and put Pass/Fail and the time it took to run the test in a common location which might just be a spreadsheet, database or formal tool.
5. Update Feature spreadsheet as components pass the user testing

Here are some example files you could use to get started.

UserTest-Results-Template.xlsx

xx-UserTests-componentname.docx