You want to start with knocking out a business model canvas. Read the book actually, it's very good.
https://amzn.to/2GjjcCH
Business models in this way are very easy to build out, but work on the framework of lean startup. Helping you build out your business model and answering any assumptions you might have along the way. They get you to test interactions and validate what you have, so when it comes to answering the questions, you have concrete answers. For example; you might think in this ABC business model that your customers are 18-35yr olds, and you can reach them through facebook as a customer channel... now, how can you validate that assumption? You can run some mock adverts to analyse the demographics of the audience, or you can bust out a survey to people on the streets and get results that way. Either way, at the end you can show figures around the assumptions you have made. So if someone asked you "who's your target market and how do you plan on reaching them" you can say these people and using that, I know this because we surveyed over xyz number of people worldwide and 80% said bla.
I'm a big fan as you might have guessed. Happy to help. It can be used for new or existing businesses and for whole new models or just changes in existing ones.
I ended up in hospital this afternoon as I was having pains in my chest and **** breathing. I was hoping I wasn't going to drop down dead, and I didn't so that's OK. Heart is fine too. Just stress. Blood pressure is a bit high so need to just sort out some exercise and try not to be stressed... which is easier said than done.