Paint shirts or clothing in general, by hand, it is not difficult in principle. The materials needed are: brushes of various types and sizes, textile water-based paints, templates, hair dryer, and a normal iron. They exist in different market ready-made templates that you can use if you like the patterns or design they represent. I never use because I prefer to do mine. In any case, do not use templates as an end result, but as schematic diagrams from which go painting over.