Prepare your station. If using nori, use scissors to cut it into thin strips, about 1/2-inch wide.
In a small bowl, mix the 1 cup (240 ml) of water and 2 tablespoons of rice vinegar to create 'tezu' or hand water. This will prevent the rice from sticking to your hands.
Dip your hands in the tezu water. Scoop about 2 tablespoons (30 ml / 1 fl oz) of sushi rice into the palm of one hand. Gently cup your hand and use the fingers of your other hand to press and shape the rice into a small, oblong mound, approximately 1-inch by 3-inches.
Add the toppings. Smear a tiny dab of wasabi paste onto the top of the rice mound, if desired. Carefully place one piece of your prepared vegetable topping over the wasabi.
Secure with nori (optional). If you like, wrap a strip of nori around the middle of the nigiri to secure the topping. Dab a little tezu water on the end of the nori strip to seal it.
Repeat the process with the remaining rice and toppings.
Serve immediately with soy sauce for dipping and pickled ginger on the side.
Don't squeeze the rice too hard when shaping. The goal is a pillow of rice that holds together but isn't dense or tough.