how to wire 2 pole 240 v contactor/120v coil
Whoops, you blew it.

Having done a very similar thing with a timer for a hot water heater, you hoisted yourself by your own petard by buying a 120V coil.  

There is nowhere to obtain 120V on your 240V device.   There is only Hot L1 and Hot L2 present.  To get 120V, there'd need to be a neutral wire there.  It's just not there.  (and no, you can't bootleg ground). 

You need a 240V-coil contactor for this.   Or, if that's a problem for the float switch, then get 24 volt low voltage from the furnace or doorbell transformer, and use a 24V-coil contactor.