To make a custom inspector, extend “Editor”, and put the @CustomEditor attribute before the class declaration. The attribute tells Unity which class (e.g. which of your MonoBehaviours) that this Editor will work on.
It looks like there’s an error in the doc link you sent. In the @CustomEditor attribute it should be the class you want to edit, not the class that you are defining below.
i.e. instead of:
@CustomEditor(MyPlayerEditor)
class MyPlayerEditor extends Editor {
it should be:
@CustomEditor(MyPlayer)
class MyPlayerEditor extends Editor {