Slice the avocado lengthwise into 1-cm (½-in) strips, place on a flat dish and drizzle with the lemon juice to prevent discoloring.
Place a bamboo mat in front of you, strips horizontal. Place 1 sheet of the nori on the mat, shiny side down with the edge of the nori 2 cm (¾ in) away from the edge of the mat closer to you.