Dear Sir
Refund can be claimed online only upto the period of two years from the date of making payment. In your case, since the payment was made on 20th oct 2018, there is still some time left for you to claim refund , on the expiry of said period one has to approach the department for the same.
Documents required:-
1. File application for Refund RFD-01on GST portal.
2. Refund applications made on account of excess payment of tax must include a statement showing the details of the amount of claim on account of excess payment.
3. Attach cash ledger
4. Challan if wrong head payment done.
5. Covering letter to mention all attach document.
6.Any other document if officer asked.
Thanks & Regards
Shiv Kumar Agarwal