how to texture painting in blender
before you texture paint, you need proper uv mapping:
learn how to correctly unwrap a 3d model in blender,
texture painting is a great way to detail to your models directly in blender, some may prefer using a 3d party application like photoshop or inkscape but blender has a robust painting tools that work in 3d and 2d, the video gives a good explanation on how to texture paint in blender.
One key note to remember is: