I guess you can start by looking at examples from other liveries that already has dirt in them. This is one example:
If you want to use the wing texture from them, you have to contact the designer for permission to use the dirt. And just apply it to your own wing texture…
It’s not my personal preference to see dirt like these. Makes me question the airline’s maintenance and cleaning activities. I prefer to fly the A320neo like it’s brand new in its maiden flight that was just delivered from Toulouse. But it is my personal preference, I know a lot of people don’t share the same preference as I do. But I hope this will help you add some dirt to your liveries.