Login with your Service Tax assessee on the ACES portal, get the Provisional ID and the Password by clicking on the link on the homepage after login. Use this Provisional ID and Password on GST portal for creating your Username and Password. You will get Username and Password through OTP Verification process. Subsequently, you need to fill in the application information and upload documents required as per the GST portal, and submit the application either using Digital Signature Certificate or using Aadhar OTP. If all things are proper, and application is submitted successfully, you will receive mail containing ARN.