Hi, I am an undergraduate student attempting repeat measure multilevel modelling for my dissertation and would really appreciate help in producing the code for my project!

I am studying protein intake of high school students and seeing whether breakfast, lunch or dinner provides the most protein. I'm planning on doing this through regression using the overall daily protein intake.

I have collected their intake over two 24 hour periods (therefore have repeated measures data.)
My fixed explanatory variables are mealtimes (breakfast, lunch, dinner) and my levels are level 2 (day of observation (either first or second 24hr period)), level 3 ( Pupil Number) and level 4 (School).
My fixed covariates are sociodemographic variables such as age, gender however I'm struggling to understand how I fit these into the model

Currently i'm doing:
xtmixed TotalProtein_ Breakfast_ Lunch_ Dinner_ || Sch: || PupilN: || DayObs:

but i don't know how to account for my fixed covariates- any help would be VERY appreciated.