To sign now, form both hands into the ASL letter Y sign, with your thumb and pinkie fingers extended and your three middle fingers curled in on each hand. Starting with your hands up, bring them down to your hip level.
Now is signed just like today.
As your toddler advances in signing, you can introduce the dimension of time. You can use signs like now and later. "Let's have a bath now," or, "Let's go to the park later."
Now is an abstract concept that you teach your toddler to introduce the concept of time. You can talk about going to the movies now, or going to the playground later.