Broil the eel on crinkled aluminum foil for 10 minutes per 1" of thickness
Make a ping-pong sized ball of rice.
If you like Wasabi, place a bit on the tuna/salmon
Place Tuna or Salmon on top of ball (wasabi to the rice) and press firmly into rice ball - on the top and sides and then press the rice sides so they are more "rectangular" than round.
For the eel, you will do the same as above (but no wasabi) and then wrap the Nori perpendicular to the eel with the seam side on the bottom.
A few additional notes, if you are making your own wasabi, make sure that you take the powder and add enough water to make it into a thick paste (still a bit powdery, not runny) and that it will only keep about 3 days.
When you dip your Nigiri, dip it into the soy sauce fish side down.