The simplest way to incorporate FFTEASY into your program is to compile and link the file ffteasy.c containing all of the FFTEASY functions into your program. Then all you will need to do is include declarations of the FFTEASY functions you wish to call. The declarations are collected for convenience at the top of the file ffteasy.c, from which you can paste them directly into your program.
If you want you can instead copy the FFTEASY functions out of the file ffteasy.c and include them directly in your code. If you do so please remember to include the credits/legal paragraph along with the code. Also, you will need to take account of the following dependencies:
When calling the FFTEASY functions bear in mind that they all take an array of numbers and replace it with its Fourier transform. In other words the transform is done ``in place,'' overwriting the original data. If you want to keep your original data you can: a) make a copy of it before doing the transform, or b) use FFTEASY to do the inverse transform when you're done, thus restoring your original data up to small numerical errors.