How it works
When you sell to businesses, an ICP describes a company: its industry, size and location, what it uses now, and what usually sets off a purchase, such as a new hire or a funding round. When you sell to people, it describes their situation instead: their job, their stage of life, what they're trying to fix.
An ICP isn't a buyer persona. The ICP says which customers to go after. A persona describes the person inside them you'll talk to, with their job, worries and goals. Most teams write the ICP first.
Build it from your own wins. List your last 20 customers, mark the ones who closed fastest, spent most and stayed, and write down what they share.
Worked example, with made-up figures: of your last 20 deals, 8 came from companies with 20 to 100 staff and a field sales team. Those 8 closed in 30 days on average and the other 12 took 75. You won 1 in 3 of the first kind and 1 in 8 of the rest. So your ICP is companies of 20 to 100 staff with a field team, and a prospect list sorted by it puts those calls first.
Check it again after your next 20 deals. The customers who buy best change as your product and your market do.
Where it shows up in your week
A poor fit shows up as deals that drop out early. The sales funnel shows your rate at every step from appointment to signed deal, counted from what you log, and names the weakest one.