Cook the rice according to the package instructions.  Once it is cooked, allow to cool down to room temperature. Stir in the rice vinegar and mix well.

Bring a large pot of  lightly salted water to a boil. Add the shrimp, cover the pot with the lid and immediately remove from the heat. Allow to stand until the shrimp is cooked through, about 6 minutes. Transfer the shrimp to a bowl of iced water to stop the cooking. Drain.

Cover the sushi mat with plastic wrap. This will prevent the rice from sticking  to the bamboo. 

Take a nori sheet  and cut in half, using scissors. Place one half in the middle of the mat with shiny side down. You can use the entire nori sheet to make a larger roll.

Keep a bowl of water nearby. Drench your hands and take a handful of rice. Spread about 3/4 cup of rice in a thin even layer. Don't press the rice much! 

Spread about 6 teaspoons of  tobiko (masago) over rice in a thin layer and flip the nori over.  The rice side is down and nori is facing up.

Place 4 shrimp, 2-3 slices of avocado and 2 sticks of cucumber  on top of the nori.

Roll the bamboo mat from the near edge  away from you. Roll firmly, but don't apply too much pressure because the filling may come out of the sides. Remove from the mat and cut into 6-7 pieces with a WET knife. Repeat with the remaining nori sheets.