If your main goal is to visit Christmas markets, pack clothes that are warm and comfortable because you’ll be outside for hours at a time in freezing temperatures.
Germany gets cold in the winter. It’s as far north as the southern parts of Canada, and there could be snow (or rain, unfortunately) in December.
Don’t go crazy with make-up because half of your face will be covered by your hat and scarf anyway. And I definitely recommend moisturizer, lotion, and lip balm to protect yourself from the drying weather.
I wouldn’t bother with jewelry for this type of trip because you’re going to be spending so much time covered up by your winter gear. A cute pair of earrings might be a nice touch if you don’t like going completely without jewelry.
Germany is a varied country with differences in dialect, food, and culture from one end to the other. This makes for a diverse experience if you decide to visit Christmas markets in several different parts of the country.
Do a search for the city you’re going to along with “Christmas markets” or “Weihnachtsmärkte” in German and you’ll find a list of locations, dates, and times. In general they run for about four weeks before Christmas.