First, navigate to the campaign where you'd like to connect the Snapchat Ads integration, then click "Integrations" from the menu on the left side of the screen.
Click on Snapchat Ads in the list of available integrations.
Then click Connect New Account.
Next you will be asked to log in using your Snapchat Ads credentials.
You will then be prompted to select the customer account which you would like to connect. Click the radio button next to the appropriate account, then click "Save".
If the account that you would like to connect is under a different Snapchat Ads account, and you need to enter different credentials to access that account, click the button that says "+ Connect New Account", then enter the appropriate credentials, and step through these screens again.