To create a Custom Audience in Facebook:

1. In the Audience section, go to Custom Audiences and select Create New
2. Select Custom Audience from the dropdown
3. Select a source
4. Enter criteria for your ad set's Custom Audience
5. Add Name for your audience
6. Select Create Audience
7. Click on Done

To find your saved audiences:

1. Go to your Audiences
2. Click Filters
3. In the dropdown menu, click Saved Audience

Custom audiences help you reach your ideal audience by entering relevant keywords, URLs, and apps. For example, you can set up a custom audience in your display, gmail, and video campaigns by simply adding specific keywords, URLs, and apps related to your product or service.