Furnace Connect G Wire
You need to add a relay to the system to independently operate the fan separate from the heat.

The simplest way is to buy a new high voltage 120VAC (20-30 Amp?) heavy duty relay/contactor that switches with a 24VAC control voltage and connect both relays to the high voltage circulation fan. If either relay is on the fan gets power. The existing fan relay will power the fan when you call for heat, and the new fan relay will power the fan when you call for fan only. 

If you don't add a relay and still somehow hook it up, your heat will probably turn on every time it calls for fan only.