If said person was truly that special, I'd make my actions and words have her understand she's the most important thing to me. Make her feel like she's the best person in the world. Put her before me, because I know that she would put me before herself when the time arises. Hopefully it's pretty balanced, however, else then there's trust issues. Help her with her goals or desires when possible. Be generous towards her. The most generous thing you can do isn't necessarily the physical amount you give, but how much of your time it took to give that thing (and hopefully that turns out to be meaningful).
How would I do this? Well I'm not sure, I don't have a special someone ! I'd certainly be tested with time in that scenario... I guess even basic things like listening to her, laughing at her jokes even if they aren't funny (or boo it jokingly because why not , not that it's usually the girl telling the bad jokes ), randomly say or do something nice, etc... just general stuff.