Start with your strongest customers
Look for patterns in the customers you serve well: their business model, buying triggers, decision process and the problems you solve. Separate these patterns from assumptions about who should buy.
Describe the buying situation
Company size and industry are a start. Add the operational challenge, the person who owns it and the event that makes it urgent. This turns a broad segment into a usable prospecting brief.
Make qualification consistent
Agree a small set of fit criteria with sales. Record why opportunities progress or stall, then revisit your profile as evidence accumulates. An ICP should evolve with your market.