How to make leaf twig, and bark textures for unity trees?

Hello, I am currently trying to make some trees for a project. Making the actual mesh in unity is easy but I am pretty bad with textures. I want to know how you would go about getting textures for things like leaves on trees in unity, or textures for grass. I know I could find some textures that would work, but I want to know how to make them myself. Any advice? Thanks