5 Types of Nigiri
Step 1: Trim fish
You need an extraordinarily sharp knife to cut fish properly. If you've got a yanagi (Japanese sushi/sashimi knife), use it. If not, a very sharp chef's knife will do.
Start with a block of fish about 2 inches wide, 2/3 of an inch thick, and 8-10 inches long. Most sushi/sashimi fishmongers will sell you fish in blocks like this.
Trim off the bottom left corner of the fish, holding the knife at a 45 degree angle to the block, and at a 45 degree angle from the board.
Save this trimmed portion for tartares.
Step 2: Start Your Slicing
Keeping your knife at the same angle, cut the fish again, about 1/4-inch away from the initial slice. Slice using a single long, smooth stroke—do not saw back and forth, as this will damage the flesh.
Cut until you are about 3/4 of the way through the fish, then...
A Finished Slice
When sliced properly, your piece of fish should come out with a minor concave indentation on one side, with no distinct lines in the cut, which would indicate sawing.
Repeat this process until you've sliced the entire piece of fish.
Step 4: Slicing White Fish
Flat fish such as fluke or flounder have tougher meat than larger round fish and thus require thinner slices.
To slice fluke, hold your knife against the fish at a much more extreme angle—30 degrees or so. Cut it into paper-thin slices using the fingers of your non-knife hand to help keep the slices intact.
These slices are thin enough that there is no need to perform the knife-angling trick to create a cup shape in the meat.
Step 5: Get Your Mise On
Fill a rimmed dish or pie plate with ice, cover it in plastic wrap, and set your sliced fish on top to keep it chilled while you form the nigiri.
You should also have on hand your sumeshi, a cup of vinegared water for keeping your hands moist, a small dish of wasabi, and if you are using them, strips of nori for tying certain rolls.
Step 7: Rotate and Repeat
Pick up the rice ball with two fingers, rotate it 180 degrees, and repeat the cupping and pressing steps. Flip it over and repeat again, then rotate it and repeat once more.
Continue doing this until either you are completely satisfied with the shape, or your guests have gotten bored and gone home.
Step 8: Wasabi
This is the only stage where my method differs a little from tradition.
Normally, you'd apply the wasabi to the underside of the fish before placing it on the rice. This requires you to either a) cup your perfectly formed rice ball in part of your hand while you hold a piece of delicate fish in a different part and use your other hand to pick up the wasabi, or b) grow a third hand.
Much easier is to simply apply the wasabi directly to the top side of the rice ball with your finger.
Go easy. A little dab'll do ya'.
Step 9: Apply Fish
I wish there were more times in life when I could use the phrase "apply fish," but I can't think of too many.
Anyhow, this is the time to apply fish. Add your fish with the concave side down on top of the rice block so that it hangs over all sides, but mostly on the back.
Use the cupping/two finger motion again to press the fish firmly to the rice block.
Step 11: Serve
Serve the nigiri immediately. Traditionally, single varieties of fish are served in pairs of nigiri, though it's perfectly acceptable to serve single pieces on a mixed plate for individual diners.
Remember—when consuming nigiri, dip the fish side in the soy sauce so that the rice doesn't fall apart. Consume the whole thing in one or two bites, without putting it down between bites.
Also, this is a hands-on experience. Forget the chopsticks, and dive right in.