Roast the rough, dull side of a sheet of nori over a high flame, being careful not to bum it, until it turns from a purple, black color to green. Place the sheet of nori, with the smooth shiny side down, on a bamboo sushi mat. 

Wet both hands with a little cold water and spread the cooked rice evenly on the nori. Leave about 1/2 to 1 inch of the bottom part of the nori, the part closest to you, uncovered by rice. Similarly, leave about 2 inches at the top of the sheet, the part farthest from you, uncovered by rice. 

Slice the carrots into lengthwise strips about 8 to 10 inches long and about 1/4 inch thick. Place the strips in a small amount of boiling water, cover, and boil for 1 to 2 minutes. Remove the carrots and place them on a plate to cool.

Place the scallions in the same boiling water, cover, and cook 1 minute. Remove and place on the plate with the carrot strips. 

Take a small amount of umeboshi paste and spread it evenly across the width of the rice about 1 1/2 to 2 inches from the bottom of the sheet so that it is almost centered on the rice. Use the umeboshi paste moderately, as it's salty. 

Next, take two carrot strips and lay them on top of the umeboshi paste or plum. Then take two scallions and lay them on top of the carrot strips.

You should now have umeboshi, carrots, and scallions lying in a straight line across the width of the nori. 

Use the bamboo mat to roll up the rice and nori, pressing firmly against the back of the mat with your thumbs, and tucking and rolling forward with your fingers on the nori and vegetables. Roll up until you are about 1 inch from the top of the nori. Wet your fingers again and moisten the end of the nori across the entire width. Continue to roll into a round log or cylindrical shape. The vegetables should be fairly well centered in the roll. Roll the mat completely around the roll and press firmly but gently to seal the nori together. The ends of the rolls are uneven as you can see by looking at either end of the sushi mat. To make the ends even, so that you get eight equal-sized pieces of sushi out of each roll, take a teaspoon of rice and pack it into the ends of the roll. Remove the sushi mat from the roll and set it aside. 

Wet a very sharp knife and slice the roll in half. Next, slice each half in half. You will now have 4 quarters of a roll. Slice each quarter in half, so that you have 8 equally-sized pieces of sushi. 

Arrange the sushi rounds (or maki) on a serving platter with the cut end facing up, showing the rice and vegetables.