For downloading the GST registration certificate (Form GST REG-06),
1. Login to the GST website first: https://gst.gov.in
2. Click on Services tab
3. Lick on "View/Download Certificates"
Now, choose to download the certificate. It would be in pdf format. The certificate needs to be displayed prominently. It has 3 sections usually,
1. Registration certificate
2. Details of additional places of business
3. Details of whole time directors or partners.
Good luck with your business!!