Step 1: Select Supplier list then select View beside the supplier 

Step 2: You will now see your Associated Sites that are supplied by this supplier or just one site if you only have one site on the system. These sites below were selected at the adding a new supplier stage

Step 3: Select Add Associated Site

Step 4: Select Site from the dropdown then select Add

Step 5: Go through and select what necessary details of this supplier that relates to your site. Once done select Save Associated Site

Step 6: Your added associated sites will look like this below