Do I need to heat the thermostat before putting it in

Hello, Not at all. I do not mean to sound condescending, but where did you hear this? The thermostat will heat on its own once the engine is turned on. The only reason to heat a thermostat out of the vehicle is during cooling system diagnostics. If you suspect the thermostat is opening and closing at improper temperature, it can be heated with water to test the specific opening temperature to that thermostat in particular.