Whatever you want, but you can just follow a basic script. Something like:
"Hi,
My name is [Name] and I'm a constituent of Sen. [Senator Name]. I wanted to leave the Senator a message.
I wanted to let the Senator know that I'm strongly opposed to the nomination of Sen. Sessions as Attorney General. His past, recorded racist remarks and actions towards minorities makes him unfit for the office, and I find it wholly unacceptable. I expect the Senator to address these issues in the hearings and oppose any efforts to confirm his nomination."
It doesn't have to be anything grandiose. Just to the point, your position, and reiterate that you're a constituent and are following the matter. Hold your officials accountable and act.
This is not okay.