make up means combination of things that form it or forgive each other (e.g. Old people over 60 made up half the population of the city. We were angry last night, but we made up at breakfast.) make someone up means apply cosmetics to (e.g. My sisters made me up for my graduation party.) make up something or make something up means invent something that is not true (e.g. My mom was always really good at making up stories.)

mix up means confuse because of a mistake (e.g. There were too many things mixed up, I was almost crazy. I always mix those two brothers up because they look so like each other.)