Spectral Analysis of EEG in EEGLab

Dear Matlab Users,

I'm conducting analysis of EEG data in EEGLab. I would like to conduct spectral analysis of power in different frequencies for certain periods of my experiments, for example for 30 seconds before attention task. I tried to find a way in EEGLab tutorial but I have only found it for ERP. Could anyone give me a tip about how to do it in EEGLab?

Thank you for your time and knowledge.

Sylwia