Hi Saad,
Thats going to be difficult top explain in a post.
Let me share this link with you as a good starting point:
https://www.cbp.gov/document/guidance/cbp-modernizes-electronic-refund-enrollment-process
Also, check out this FAQ on the CBP website:
https://www.cbp.gov/trade/automated/ace-portal-and-ach-refunds-faqs
Good luck getting started.