Cook the rice according to package directions. While you can absolutely make a good sticky sushi-style rice in a pot, I think a rice cooker is the best option if you have one. When the rice is still warm, but cool enough to handle, sprinkle the rice seasoning over it and then gently fold the rice with a spatula to incorporate it.
Mix the rice with mayo (or not) and any of the optional additions and season with salt, to taste.
Divide the rice into 8 equal portions, form a round of rice with a dent in the center, then fold the rice around the tuna. Form the rice ball into a triangle, then wrap with a piece of nori. Repeat with remaining ingredients and serve immediately.