how to design a filter in matlab without any toolboxes (only fft)?

My task is the next: Load a wav file, then design a filter according to a pre-defined amplitude characteristic (the amplitude characteristic are any example, For instance: a series of numbers between 0 and 1 along the spectrum). To solve the task i must use only fft without any toolboxes. Please help me

[Jan]

You forgot to mention what you have done so far and which problems have occurred.

If you are supposed not to use toolboxes, I'm sure asking a forum to post a solution is not accepted also. So please ask a specific question and show any own effort.