I did this on mine a long time ago, but I had Megasquirt to use as a programmable output.
The fan control relay has 12V+ and ground going to it, and there are two 'control' wires. These are grounded to turn the fan on. One for 'low' speed, one for 'high'.
My car was a late model 240, so it had a pusher fan (that I removed as part of this mod) that came on when the A/C was on (and hot, I think it was hooked to a pressure switch in the system, not merely on when the compressor was on). I kept that system in place, and added an MS output to that, to control the 'low' speed setting. In MS I set the 'on' temp about 5 degrees hotter than the thermostat temp, so in normal use with the car moving, the fan wouldn't come on at all. There's no reason to run it at the thermostat temp, it will just run constantly.
The temp sensor in the radiator was set (from the factory?) to a rather uncomfortably hot temp - it came on at something like 220 degrees. I used that to engage 'high' speed on the fan, as a 'dumb' failsafe. Just ran a wire from the relay 'high' terminal to one side of the switch, and a wire from the other side to a ground.
On the low speed circuit, MS already was set up to ground a circuit to engage the fan, so I just connected the wire up to the relay. But the existing pusher fan circuit switched on 12V+ to engage the fan, so I just used a cube relay there.
'Control' circuit:
pusher fan circuit 12V+ --> relay terminal 86 --> relay terminal 85 (although really, 85 or 86 are interchangeable) --> ground
'Main'
low speed relay wire --> relay terminal 30 --> relay terminal 87 --> ground
That turned a 'hot is on' pusher fan signal to a 'ground is on' cooling fan signal.