What do I include in a letter to my state senator about a bill she co-sponsored that I deeply object to?

I know I should make clear that I’m a registered voter in her district, tell her I object to the proposed bill, and tell her why, but what is the action I should ask her to do? Do I say, “Please reconsider your support of this bill” or something else?

I should also write to my other state senators who didn’t co-sponsor it. How can I find out what their position is? If I don’t know their position what should I say? “Please don’t vote for this”?