in my C class for engineering, i'm having problems understanding how to make/put functions on the program

these are my teachers notes
http://ringer.cs.utsa.edu/~mmaltrud/.../functions.htm

i already had 2 homeworks using fucntions and well.. the first one i got a 50 caz i did it wrong and in the second one a 70.. and i have a test on april 19th so i want to learn how functions are created

i kind of have the idea of what the idea is, but when doing a 'recall funtion' or 'return function' i just don't get i do't know where to put them or what variables to put

care anyone to spend time in teaching me
thanks