Creating a guest wifi in Tomato is easy. Just follow these screen shots: To begin:

1. Login to the Tomato Control Panel

2. Click Network under basic settings and add a new lan connection as the screen shot shows:
   

3. Go to VLAN under advanced and create a new one linking it to the new LAN. You will have to restart the router after this operation.

4. Add a new virtual wifi interface

Job done!