What every you do, sell or service, you need to work out HOW you are going to do it. Some people call it the business model or business process, but in simple terms, you need a step by step process for how you will make your money.
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Example

A 4 person Plumbing Business 'could' have the following process: 1 - first contact with customer, provide prices for call out and organise appointment 2 - Assign customer to plumber. 3 - Plumber visits and provides quote to Customer. If customer agrees, work is completed on site, or new appointment is made. 4 - Invoice is sent 5 - Payment checked and if not paid, chased.

Now consider other possibilities

What happens if the customer does not pay? what process will you use? What if a plumber calls in sick, or is help up longer at a job. What should happen if a customer is not at home at the appointment time? All these situations should be considered and you should have a plan of attack to handle them.