On a flat surface, place the mold cutting guide over its base to create an rectangular molding box.
Place a sheet of nori inside the cutting guide with the grainy side facing upward. Gently use your finger to pull the nori down and indent inside the molding box without breaking it.
Fill molding box to with sushi rice to about 1/3 of its depth. Use your fingers or small spatula to spread the rice evenly.
Spoon in the spicy tuna filling, then arrange cucumber and avocado slice.
Fill the box with sushi rice to cover all the fillings.
Gently tuck to close and seal the nori ends by dabbing rice vinegar-water to adhere (1:1 ratio).
Now choose the heart shape lid to place over top maki roll. Press and apply pressure evenly to the lid to mold the sushi roll beneath.
Carefully remove the lid (without breaking the roll). Use a sharp dedicate knife to cut sushi roll follow the cutting guide.
Replace the top lid back onto the roll. Apply some pressure just in case your roll get shuffle during cutting.
While pressing the lid, carefully remove the cutting frame.
Flip the roll along with lid/base upside down, so now the lid with be on the bottom and base on top.
Gently tap the base a few times to release sushi pieces.
Carefully handle and arrange these pieces on serving dish.