This tutorial is for users who already have a Synccentric account and want to authorize Synccentric with SP API. Login to your Synccentric account and go to your Campaign settings (‘Manage Campaigns -> Edit or Create New’ ):

Select the locale you want to authorize for and input your 'Seller ID', then click 'Authorize' button:

You will now be directed to your Amazon Seller Central Account to Authorize Synccentric as an app. Once you login (or if you're already logged into Seller Central), you'll see an authorize page, click the checkbox and continue authorizing:

You'll now be redirected back to Synccentric. You can check to confirm our app has been authorized both from your Seller Central account and from your campaign settings in Synccentric.