To create a function, you need to define a name, any input arguments and their types, and a return type.
Suppose fastBlink needs to know what pin to blink, how many times to blink the pin, and how long to wait between blinks. There is nothing that the function does that the caller needs to know about. The return type is then void:
void fastBlink(int pinNumber, int blinkCount, unsigned long blinkTime)
// Write your code here
Each variable has a type, and a meaningful (to me, at least) name. Given this skeleton, actually writing the code should be quite easy.
Creating a function for the Arduino is no different from writing a function in C for any other computer, which is why the Arduino documentation barely mentions it.