Develop theme for wordpress to upload to themeforest

I have a great experience in developing WordPress website but now i want to develop the theme so that i can upload it to the theme-forest and other user will download my theme and use it.
any suggestion or help ? how i build the theme for WordPress core functionality i know.
Next question is can i use underscore theme and after that i design it beautifully then i upload to theme-forest ?

I will be happy if anyone can help me in theme development guide.