How do you do map extensions in D365?

I am looking at https://docs.microsoft.com/en-us/dynamics365/unified-operations/dev-itpro/extensibility/map-extensions and https://docs.microsoft.com/en-us/dynamics365/unified-operations/dev-itpro/extensibility/maps-as-interfaces, 

1. how did the author create the SalesPurchaseTable.ISV2Extension from SalesPurchTable map? When I try to right click SalesPurchTable and try to create a extension in existing project, nothing happens. I tried also tried to create a new project to do it, still nothing showing. 

2. How did the author add the method validateAccountingGroup() in the ISV2 table extensions? You can't add methods in extension tables.

Tkx in advance