There’s a tutorial on the BeamNG documentation site: https://documentation.beamng.com/modding/materials/vehicle/skinsystem/skintutorial/ You have to follow the steps carefully and pay attention, it is kinda vague at parts. I used this for a skin for my mod (the skin is still unreleased though) People also made skin tutorials, so maybe you can use that.
png skins work the same way as dds skins do. the game will automatically convert them to dds though because the format is optimised for loading times. just make sure your filenames are ".color.png" so the colours convert properly
