STEP BY STEP INSTRUCTIONS FROM FACEBOOK'S WEBSITE:
A Lookalike Audience is a way to reach new people who are likely to be interested in your business because they're similar to your best existing customers. Read more about Lookalike Audience .
Before you begin
To create a Facebook Lookalike Audience, you must be the admin of the Page or pixel you're creating it from.If you're making a Lookalike Audience from a Custom Audience, you must be the creator of that Custom Audience.If you're creating a Lookalike Audience from mobile app installs data, you have an Administrator or a Developer role on the app.
Create a Facebook Lookalike Audience:
Go to your Audiences Click the Create Audience drop down and choose Lookalike Audience Choose your source
Notes:A source can be a Custom Audience not created with your pixel data, your mobile app data or fans of your Page Consider using a group of 1,000 to 50,000 of your best customers based on lifetime value, transaction value, total order size or engagement.Choose the country/countries where you'd like to find a similar set of people Choose your desired audience size with the slider. 📷
Click Create Audience You can also create a Lookalike Audience during ad creation in Ads Manager.
Keep in mind:
It may take 6 to 24 hours for your Lookalike Audience to be created. After that, it'll refresh every 3 to 7 days as long as you're still actively targeting ads to it.
You don’t need to wait for your Lookalike Audience to update to use it for ad targeting You can see when your Lookalike Audience was updated by visiting your Audience Manager page and looking at the date under the Availability column