The first step is to create a new object with the public IP address of the other side of the tunnel.  This is fairly well buried in the menus:

R80_30_new_VPN_interop_device

After that, create a new VPN “community” in Objects -> More object types -> VPN Community -> New Meshed VPN and walk through the wizard.

The main gotcha is watch out for weird default settings.  In particular, AES-128 is disabled as encryption cipher for Phase 1.  My guess is since it’s the most popular cipher for Phase 2, they go with the “mix ciphers” strategy.  But personally I just like to use AES-128 for everything – it’s simple, fast, and plenty secure.