SMPS gain and phase

Hi, I am trying to simulate gain and phase response for stability on a DCDC buck circuit. I created a custom plot window with 2 probes across the AC source. I have tried applying AC signal in the feedback line to the DCDC IC or at the top (Vout) side of the feedback resistor network but neither are giving expected results. Can anyone point me to a tutorial or guidance for getting this to work please? Thanks.