Procedure: The steps to do MTDVAT authorization setup is as follows:

Step 1: Go to our application MTDVAT and Sign in to the account. If you don't have an account, click on SIGN UP to create an account.
Step 2: After logging in, select the hamburger option on MTDVAT's menu..
Step 3: Next, click on the "Client / Trader Manager" submenu. This will take you to a page that contains all the information related to clients/Traders.
Step 4: Now, click on the ‘View /Edit’ button on the page of the client/trader that needs to be authorised.  
Step 5: Click on the MTDVAT Authorization button on the same page. It will take you to the MTDVAT setup page. 
Step 6: Once you have clicked on the 'MTDVAT Authorization' in the MTDVAT Setup, the page will take you to the Government Gateway page where you will need to enter your credentials to grant authorization..
Step 7: Once you've filled out your government gateway credentials like(User ID and Password), it'll take you to a page where you need to click on the "Grant Authority".
Step 8: Once the authorization has been granted, the VAT information will be collected and submitted to HM Revenue and Customs Administration (HMRC) on your behalf.

** When the Authorization process is completed, you will be able to submit the MTD VAT e-return using a "CSV or XML" file**.

RoadMap for MTVAT Authorization:   "Client/TraderManager" -> Click on the View/Edit  of Client/Trader -> Select  "Mtdvat Authorization" -> Enter your "Government Gateway" credentials -> Grant Authorization...

Here's a video on how to setup an Authorization process for MTDvat: