question about authenticating a users on different forest domain.

Hi All

I have a network setup as below :

if we want to authenticate the users from adatum.com on VPN1, what should you do?

first of all, i configured NPS1 as nps server and VPN as Client server .. computers from internet where able to connect to 192.168.1.10 ,

then

I configured nps2, as vpn and as RADIUS client

i tried to create a network connection policy on NPS1 , in the condition part i didn't know what to do ... could you please show me in steps what  i need to do and why we do these ...

my purpose is learning

many thanks.

regards