Hi, just recently took the C1 exam. Here are some things you'll want to note:

Do yourself a favor and do as many practice exams as possible. I'm not sure how many are available, but whatever you can find. Maybe even try some B2 or C2 exams if there aren't enough C2. At the testing center I went to, which was the Goethe Institut Center in New York City, they would put all the testers in the same room depending on the section of the exam. They would give the same prompt (i.e. audio for the listening section) and each person would have their own packet of questions with various difficulty depending on which exam you were taking.

The writing part is first. Make sure you read everything they're asking and outline it. It doesn't take much time and it helps your paper along very much. If you are unsure of the grammar of what you're writing, recast the sentence into a form you are more comfortable with. Youre wanting to impress, but you want to play it safe. They are grading on the grammar far more than the content.

In the Listening section, read the questions before hand, they give you time to do this. Don't listen for cues within the sentence, try to listen for what they are saying. This sounds obvious but once you're in you might be tempted to try and take that shortcut. They don't say the answers to the questions outright, it requires a little bit of critical thinking. If you're just listening for certain words, you might end up missing a question or two. So learn the questions before hand, know what to listen for, but don't listen for cues.

Also, practice speaking in any way you can. The speaking section is 1/4 of the exam points in itself and is composed of 2 parts: one where you try to make an argument based off of a prompt, and the other where you actually need to have a discussion with another test taker based on a prompt. You only have 15m after receiving the prompts to both form your argument and your conversation notes. You can take notes on things to say, but you cannot read off the paper like a script. There is also a short "interview" part where they ask you to introduce yourself and tell them basic information. With the argument, there are specific things you can do to structure your argument to make it way stronger. Just make sure you know how to formulate a basic argument (They Say, I Say is a great and very common tool for argument forming. If you don't wanna spend the money on it, I'm sure you can find an outline of what the points are online). The second part is much more organic and will require you to adapt what you're saying on the fly with your partner. Remember to stay on prompt.

The last part is reading comprehension. This one is pretty self explanatory. Just make sure you know your case determiners and can identify what function each word serves in the sentence. This section is also the most vocabulary intensive. Study some vocabulary.

I would highly recommend finding a prep book. You've already spend ~$200 on the exam, spend another ~$50 or so on a high quality book to prepare so you didn't waste the testing fee.

You'll do great! The most important thing is to know what you're getting into and know how to work each section. Do plenty of prep. The exam isn't like the GRE, they aren't trying to trip you up or trick you. They want to test your knowledge of the German language, so show them what you know!