how to cacualte the 2nd or 3rd order nonlinear coefficences?

in volterra series analysis there is alwary the nonlinear coefficences.
I don't know how to get them?
please give me some clue!:D

input a sine wave,
caculate the THD

Added after 1 minutes:

input a sine wave,
caculate the haronmic

Sorry, I don't understand what do you mean.
please say it in a easy way, you know, I'm really a new guy.
Thanks a lot!

do some reading on nonlinear system identification or look at digital transmission benedetto and bilgieri