Badi for FB60 for validating Vendor

Hi Experts,

I got a requirement to implement a BADI for FB60 for validating whether the Vendor is foreigner or not.

The logic is as follows.

By using the Vendor Number provided on the FB60, I need to get the LAND1 from LFA1 table.

if the LAND1 not equals to 'USA', then it is foreign Vendor.

I know the entire logic to write in the code, but don't know where to write.

This is my first object in BADI. I've done some research and found some BADIs, but unable to get clear picture where to write the code.

Can any body tell me which is the BADI and where to write the code?

If anybody gives me step by step, it is very useful.