It is simple. For the national anthem, stand facing the flag of this great country with your hand over your heart and with gratitude that you are able to do this. Kneeling is for praying. Neither of these should be used for political statements. There is a time and place for everything.
If you are a sports player of note, the world is your oyster and you have great access to the press. Use it. You do not need to use the national anthem as a platform.