Today I had the thermostat changed in my 08 Infiniti and it's leaking. Any tips or suggestions?

take it back to where you got it done. if you did it then look really close to where it is leaking.

A friend replaced it.- He put it on and pulled it off twice because it was leaking. Do you put a sealant on it and is there a process to it? Someone else said you put the sealant on leave it 15 min, start the car and let it heat up to help the sealant. I know some about cars but I'm still female and don't know everything when it comes to replacing parts.

You don't need sealant. Just make sure you replace the gasket and torque the three bolts down properly (75 inch-lbs I think).

Why did you change the thermostat?
Was there a problem with the old one?
Did you use the correct thermostat?

Did you or your friend follow the steps in the FSM?

http://www.nicoclub.com/FSM/G35/Sedan/2008/CO.pdf

See page CO-22

Telcoman

Not sure about the 08 on my 06 it was a metal gasket if I remember right. You can pick up some RTV or "the right stuff", which is pretty instant.