There are two different answers depending on what you what you mean.
If you like to add a tag for a checker to an existing script you can do that directly. You have to find a script that works for the challenge.
Look under Tools/challenge checkers. You can directly search for challenges that already have a checker and use the same script och use find script and search for a script appropriate for the challenge. There exist a button named Tag script press that button and add the GC code and an appropriate config for the challenge.
Press save and try the checker. Use the verify tag to test the last 10 found logs and press use checker to test the checker on appropriate users I always tries it on my self and verify that the output is what you expect.
The hard part is to find the correct script and the config.
Look at the description of for description how to use the script and look at other challenges tagged on the script to understand the syntax. Also look at the challenges to verify that your interpretation of how the script works i correct.
If the question is how to create a new checker the answer is to Project-GC Support and ask for access. Then the problem is to understand LUA. Read the documentation and look at other script to figure out how to code new scripts
