• Go to the Catalogue tab and click on the “Add product” button
• Complete the fields for GTIN and product name (adding a group is optional)
• The GTIN field does error validation so that you cannot upload an invalid GTIN or one that your companty does not own
• (Optional) For the group field you can either select an existing group you have already created by clicking inside the product group box, or you can type in a new one to create along with the product
• Click on the "OK" button

If you have previously set up your product attributes see this article in the Help Centre.