Simply log in to your CMS backend.
Head to SEO > Analytics > Custom Tracking
You will see multiple windows.
Your code will usually tell you where it wants to go. So follow the instructions and add it to the correct section.
If your code is not working after you have added it and cleared your website cache, make sure you have Analytics tracking turned on. The switch should be pointing to the right and blue.
If all is correct you should be able to validate the tracking after a few hours.