New businesses have a greater challenge - they will rely more on market research using libraries, trade associations, government statistics, surveys, competitor observations, etc. In all cases, make sure your market analysis is relevant to establishing the viability of your new business and the reasonableness of the sales forecast.
So how would you go about making a plan like this? Well if you know a fair amount about business, you can. It will take some special calculations and some work but if you know how to put together a Profit & Loss Statement, you can probably do it.