The best way to wish someone a merry Christmas is to simply say "Merry Christmas!" You can also add a personal touch by wishing them a happy holiday season or telling them how much you appreciate their friendship or company. Other Christmas greetings include "Happy Holidays," "Season's Greetings," and "May the spirit of Christmas bring you joy and happiness." If you're feeling particularly festive, you could even sing a carol or play a Christmas tune. Whatever you choose to say, the most important thing is that you convey your warm wishes and spread some Christmas cheer.