I had the honour of spending this evening with Alissa and her mom Kaiko, and we had sushi cones for dinner - self-assembled, taco style.
The most important thing, in my opinion, is making the rice properly. Sounds simple enough, but there are many variables: how to clean it, how much water to use, how long to cook it, what flavors to add, etc. This is what she showed me:
You have to clean the rice thoroughly and quickly. Cover the rice with water, shake it/ mix it all around briefly, then dump the water. Repeat 2 or 3 times, until the water drains clearly. It has to be done quickly so that unwanted powders and odors get washed out, rather than absorbed.
I'm not sure what the water-to-rice ratio was, because we used a Japanese rice cooker with the volumes marked off. If the rice is "young" (ie: less than a year old) then it shouldn't soak prior to cooking. Older rice should soak for 10 minutes, then the rice cooker can be turned on. It's important to make the rice on the dry side, using a touch less water than recommended, so that it won't become too wet when adding the vinegar later.
The cooked rice is immediately removed from the rice cooker and spread out in a wooden (cedar) bowl - the wood will absorb excess moisture. Now the flavors are added: a bunch of sugar, a high quality vinegar, and salt (the amounts were specified on the bottle of vinegar, in Japanese). Then the rice is fluffed/ mixed with one hand, fanned with the other until room temperature.
Our fillings were: julienne cucumber, water cress, grilled salmon, unagi (eel), sour plum sauce, flying fish eggs, and negi-toro (negi = green onion, toro = tuna belly). The negi-toro is prepared by mincing tuna belly, a fatty part of the fish, and mixing it with chopped green onions.
At the table, we took pieces of nori, added some rice, added toppings, then rolled into cones. Very filling, and delicious.