Creating DB link in SAP ABAP to retrieve data from external system

Hi gurus, Hope you all are fine. I have a requirement in which i have to retrieve data from external system database (Oracle) and then need to post it in SAP FI. I have never done this before. Can anybody tell me step by step how can i create DB link in SAP to access external system tables and what ABAP code need to write for it? Kindly guide step by step. I will much appreciate that.

Thanks