You can find video as well as written down tutorials here for learning about how the hack works. You can go through the different links listed here and learn more too. Some steps have been discussed here in details. Firstly you need to put in the username of yours in the game.
After entering the username you will gain access to the server. After gaining the access, you need to put in the amount of gold or gems you desire. The higher the amount you put, the longer you need to wait for the results. After hitting the generate button, you will get Clash Royale free gems as much as you had desired.
Log in and get Clash Royale free gems
You need to have some patience and wait till your gem and gold gets generated. After the process ends, you need to log back at the end of the waiting time and find that the required amount of gold and gem has been credited into your account.
As mentioned earlier, the process of getting the gold and gems is completely legal in nature. Therefore you also require maintaining some control over the way you spend the new found wealth. The Clash Royale gems hack is loved by all owing to its effectiveness.