Cepstrum analysis: how to find quefrency step?

Cepstrum analysis: how to find quefrency step?

I'm newbie in cepstrum analysis. So that's the question.
I have signal with the length 4096 and sample rate 8000 Hz. I make FFT and
get the array with the length 4096*2 (2*i position is for cosinus coeff,
2*i+1 position is for sinus coeff). Frequency step is
(sampleRate/signalLength == 8000/4096). So, I can calculate frequency at i
position this way: i*sampleRate/signalLength.
Then, I make the cepstrum transformation. I can't understand how to find
quefrency step and how to find frequency for given quefrency.