As you might know, it was not possible to stop your AKS clusters completely because system pools are always required to be running. I have posted about this like below in past.
It should be really easy to follow the article. I have open Azure Portal and try to stop my AKS cluster like below, and it seems to be fine even if you have never registered "extension aks-preview" like below.
You can find your node pools will be 0 nodes on Azure Portal or command lines after you have completed the commands.