You may add a marketing source label when creating a new customer or you may just edit an existing customer.

In this example, let's assume you want to add a marketing source to an existing customer. First, go to Contacts > All Contacts.

Search then select the customer.

On the customer's profile, click on Actions > Edit.

On the first section, you'll find the Marketing Source field. Click on that to expand the options.

You may select the [+ Add New] option and add in your marketing channel (i.e. Google, Craiglist, Facebook, etc.).

If the marketing source is already present in the options, just click on it.

Don't forget to save your changes. Save is located under the pink Actions tab or at the bottom of the screen.

You would then see that the Marketing Source field for the customer you edited is no longer blank.

Click the underlined "Manage"

Using the crossed arrows, you can drag them to rearrange the order in which the marketing sources are listed.

You can delete unwanted marketing sources using the red trash can.