Define the seed segment
The seed is not "all customers" - it is your best customers, defined by whatever actually correlates with long-term value in your business: repeat purchase count, LTV threshold, subscription tier, or a specific high-margin product line. This requires having that value signal available in your data model in the first place, which is frequently the actual gap - not the lookalike mechanism itself, but the absence of a clean "who are our best customers" query to seed it with.
Sync it
The seed list syncs as a standard Custom Audience (Meta, TikTok, LinkedIn) or Customer Match list (Google), then each platform's own lookalike/similar-audience tool builds the expansion audience from it - this is a platform-native step, not something built outside the ad platform. Refresh the seed periodically (weekly or monthly is usually sufficient, since best-customer composition changes slowly) rather than in real time.
Why the creative differs
A lookalike audience is still a cold prospect from the platform's perspective - they have never interacted with you - so the creative should look like prospecting creative, not retargeting creative. The advantage of a well-seeded lookalike is efficiency (the platform is targeting people statistically more likely to convert at your value), not a license to skip top-of-funnel messaging.
KPI framing
Measure a lookalike campaign against new-customer acquisition cost and, where trackable, downstream value of acquired customers (not just conversion rate) - a lookalike seeded on best customers is specifically trying to buy customers who look like your best ones, so the KPI that matters is whether it actually does, not just whether it converts cheaply.
Illustrative, not a measured result: a retailer replaces an "all customers" lookalike seed with a top-20%-by-LTV seed of the same size. The expected effect of a tighter, higher-value seed is a lookalike audience that skews toward higher-value prospects at similar or better acquisition cost, versus one that skews toward an average customer profile - illustrative of the mechanism, not a measured client result.