To construct an inter-AS VPN, you would follow the same steps as those used to construct a L3 VPN, with the additional step of specifying routers from more than one AS. The following figure shows three PEs (along with completed interface assignments) from a particular AS (57301) already added.
The next step would be to choose another AS (from the AS dropdown box under Available Devices), and then select routers from it. As the following figure shows, two routers from AS 57222 were added to the VPN to create an inter-AS VPN. Interfaces were then assigned to the two routers to complete the process.