Easiest way to do it for that ‘authentic’ sound is to borrow/buy/hire a pair of walkie talkies for a few hours… put one in another room near a mic and use the other one to speak into… you can of course mess about with various EQ settings such as using a notch filter to boost between about 800khz or so to 4Khz or so and cut either side of it… just fiddle with the Q and centre of your filter… bit of distortion, compress and expand as PMRs compand the signal… and a burst of pink noise at the end of every ‘transmission’ to give you the ‘cccchhhhhkkk’ when the talk button is released… used both methods in the past but micing a pair of radios is by far the quickest and easiest method.
hope that helps!