There are a plethora of examples you can follow of high-intensity interval training. You can easily find these on a variety of sites online. They generally give you how many intervals to do and how much time to do them in. There are numerous guides about how to handle high-intensity training when you are over a certain age.
There are also many different articles and/or pieces of research that explain the benefits of HIIT. Also, online, you can find several tips and modifications on how to successfully do interval training if you are prone to injuries.