Highlight Selected Package

How can I make my own highlight selected package? What is the process?

Are you inquiring on how to make a language / syntax package?

Have you searched if the one you are looking to create is already available?
Which grammar are you aiming to create?

To which grammar would you say your version would resemble most closely?..
C++, Python, Assembler or [NSFW]

That’s what I understand for a highlight selected package