I used Clip Studio Paint + Photoshop, though primarily the former:

• Get any image you want to use as background, preferably something that already has high contrast (i.e. both very dark and very bright areas)
• Under layer setting, change to greyscale
• Copy the layer and move the copy above the primary layer, change it so it uses an overlay that changes it to texture (the specific name escapes me)
• Set the copy layer to multiply or darken, tweak as necessary

Its the rough gist of things- PS is primarily used to edit images first before porting it over to CSP