FFTEASY is a set of functions designed to be placed inside a C program. Because these functions can only be used inside a program that you have presumably written yourself, I will assume that you are already familiar with the C language. If not I'm afraid having a set of C functions isn't going to do you much good.
The next section lists the functions that make up FFTEASY and describes their arguments. Subsequent sections talk about how to integrate FFTEASY into your code and describe the storage scheme used for the Fourier transforms, including the special issues that come up in storing Fourier transforms of real data. The section on storage also describes the Fourier transform conventions used in FFTEASY.