First get a merchant account here: https://www.coin-tunnel.ml/signup-m to get an API key. Make sure to connect your bitcoin address and make your own verification string! (In your dashboard)
Your API key is only shown once! Make sure to write it down or put it somewhere safe.
Once you get an api key, that's basically it! If you are a beginner to coding, just copy our example files, and put your api key in place. If you want to really dive deep, keep on reading.