How does one go about making a single function like that?
Not sure If I understand the question. The function includes step by step the assignments you made. As its all in one function we don't have to male them them functions but just simple variables, which are dependable on the arguments of the function. So its written as it ayou may find it in textbooks or you write it yourselft - we all are sloppy now and then and forget to write out all the function arguments and dependencies. We often are not aware that we are basically dealing with a function when we write u=2*r*pi at the top and later use this "formula" to calculate the circumfence of a circle with radius 10 cm.
Or was the question meant as of how to type and create a function? In Prime its rather cumbersome (as opposed to "real" Mathcad as you have to go to ribbon menus all the the time. You have to use th programming menu in the first ribbon to acces all those pragramming keywords (of course there are keybpard shortcuts and if you are using an English keyboard, they may even work as stated.
Turning calculations into functions can be a most powerful tool and very handy and convenient.