To create an audience, navigate to your account in Google Ads. Click Tools then, under Shared Library, click Audience manager.
To create a remarketing list, click the Remarketing tab. To create a custom audience, click the Custom Audiences tab.
Click the button and follow the prompts to create an audience.
Once an audience has been created, you can add it to campaigns by selecting the audiences you'd like to add to campaigns, click Add to and then select Ad groups or Campaigns, whichever you'd like to add campaigns to.
Select the ad groups or campaigns you'd like to add the audience to, then click Next.
Select Observation and then click Add Audiences.