If using Spam, thinly slice and pan fry for a minute or two on each side over medium heat. If using leftover cooked meat, you can heat it in the microwave as necessary. If using canned tuna, press the liquid out.
Put a sheet of nori, shiny side down and shorter side towards you (the photo may not look like that because of the camera angle), on a cutting board or plate. Cut a slit to the middle of gim (seaweed sheet) from the short side.
Spread the rice evenly over the bottom left quarter of the seaweed sheet. Arrange the other ingredients on each quarter. Lift the bottom left quarter and fold it onto the top left quarter, then to the top right, and finally to the bottom right. Press lightly and tuck the filling in after each fold.