Hello
I'm not a professional. But I got these instructions from my hairdresser. I always take 2 colours and mix them. I take a little more of the colour that is more important to me. The amount depends on the length of the hair, or how much is to be covered with this colour. Usually one tube is not enough. Then you need developer = the same amount as the complete colour. Mix well. It is better to choose a generous amount. 60-90 grams is the minimum. However, I dye, tinting is much too much effort if it doesn't last long. The first time, choose colours that do not cause so much damage. Duration approx. 35 min. Good luck!

PS: If you have a good hairdresser, ask her again. Nobody wants a Glatz.
There are also good instructions on the net, YouTube, etc.