You can create your own scripting language for your game. But there may not be a "best" approach to this. Though you need to know the extent of the language. How will it be used? How complex should it be? What sort of flow or syntax will it use? As far as how to implement it, you can use GDscript. But it, again, depends on the purpose of the script. If you think speed will be an issue, go with C/C++.
See also this Stack Exchange answer to a similar question.