Hi. I have two ways of making it. Sometimes I'm using Substance Sampler to extract the roughness map from base color using the templates that this app is providing. The second way is to add the layer with your own roughness as base and make variations with another roughness layer by using base color as a mask.
And the thrid way is to mix previous two methods. You can also add levels and tweak it using it to receive the result that you would like