Once your rice is cooked, dump it onto a clean flat surface. Add rice vinegar and a bit of sugar and paddle it back and forth, cooling it completely. It's not a good idea to chill it because it become stiff, but you also cannot keep it at room temperature for more than two hours...
If you defrost an entire piece of fish, you have to consume an entire piece of fish. It's not safe to defrost and refreeze, unless you cook it after defrosting and prior to refreezing.
I don't overmuch care for nori, so I usually eliminate it when making sushi, but, that is a preference....
It helps to have a rolling mat. Line the mat with a piece of plastic wrap. If using nori, place it on the plastic. (Some people like to warm it over an open flame..it releases more flavour that way) Gently but firmly press the rice onto the nori (or plastic sheet) Line your filling onto the rice, keeping them close together. Then, lift the mat, and with the help of the plastic wrap, roll the sushi, pressing together. Use the plastic to help get it and keep it tight. If you are not going to serve it right away, chill it, uncut.
If you are going to serve immediately, cut it, doing a sushi slice, which makes a nice presentation, I think. Cut on a bias, turn the roll half way, and cut on the opposite bias, continuing till you have none left to cut.
Serve with wasabi and pickled ginger....
Now I want some.