How to connect Asterisk running behind openvpn from softphone?

I m an asterisk newbie and have created a basic PBX for calling through softphones, but i want it to run on openvpn. since i’ve installed openvpn on the same box with asterisk but now i m unable to connect my softphones with asterisk. Can anyone please guide about the configuration or either settings i need to connect my softphones in these conditions???

Open a openvpn tunnel (eg tun) between the softphones location and your asterisk, route the asterisk IP through this tunnel and make asterisk listen on openvpn net.

Can you please guide in detail???