There are a few different ways to do this, and the "proper" way is going to depend on a lot on your needs for security, your setup, etc.
In the Trust-VR, you can set the option to "auto export route to untrust-vr" through the WebUI. In the CLI it would be "set vrouter trust-vr auto-route-export." That will send the routes from the trust-vr into the untrust-vr.
Then, again in the trust-vr, you can set the "Use default route" option in the WebUI to point the default route to the untrust-vr. In the CLI that would be "set vrouter trust add-default-route vrouter untrust-vr."
If you want more control over which routes are sent between vrouters, you can use export and import rules to filter how routes are sent between VRs. The online WebUI help has a decent overview of it, or you can check the ScreenOS Concepts & Examples Guide. In the 6.3.0 C&E Guide, under Part 7 (Routing) / Chapter 33 (Routing) there's an overview and descriptions of vrouters and how they work, and near the end of that chapter is a section on importing and exporting routes between virtual routers.