How to I wire an electric furnace so just the fan runs in the summer.

In the furnace there will be a fan relay , if you have a terminel board in the furnace you will see the thermostat wires connected to this board. The "G" terminel is for the fan relay, if you connect the green wire from the thermostat to this terminel and then connect the same green wire at the thermostat you can then operate it from the thermostat.

Why is my fan in the furnace running without stopping