Most often, an ICP will include firmographics, psychographics, goals and objectives, pain points, and buying process information.
To fill these out, analyze your best customers and find the common denominators. Don’t be afraid to put ranges instead of precise numbers, just try to make up categories that make sense.
If you don’t yet have any customers, hypothesize. What type of company would be most likely to need your product or service? Then test and refine as you go.