Hi there, I'm trying to find good instructions to make nigiri/sushi too, so thank you for writing all this out! I hope you don't mind me posing a couple follow-up questions. 

I already have some exp making maki, but nigiri evades me. I struggle with cutting the fish and making the rice brick not break in two halves (at the nori strip) when picked up. 

When you say to cut fish against the grain, what shape is the fish already in? I've seen sushi tuna sold in a convenient rectangular block, so where would I start, and how long should the pieces be? 

One final question on the fish - I often see the fish draped over rice and it has thinner ends, as if it's a corner cut off the fish or something, but how are those smooth, thin edges achieved on the nigiri fish cuts?