Step 1.

Navigate to Contacts > Customers.

Step 2.

Click on the Add customer button. 

Step 3.

Make sure you are on the Basic information menu and fill in the required fields i.e Customer. Fill in any other appropriate fields.

Step 4.

Next, click on the Preset information link in the box to the left and update the This contact is drop-down box to A Customer and Supplier.

Step 5.

Hit the Save button to complete.