Look at examples of food webs, such as those found in the Resources section offered by Southeastern Louisiana University and The University of Hawai'i System. Notice how almost each animal and plant species is connected to more than one creature in the ecosystem. Also notice that the arrows point towards the consumer or producer, such as the arrow points from the mouse to the snake; this is how you should set up your Lorax food web.
Ask your instructor, if this is an assignment, whether you should strictly focus on the diet or food consumption of the characters in the Lorax or if you can approach it from a metaphorical angle. In this case, you can connect characters that affect others in ways other than actions that involve eating. The concept map on the School World website shows such an example.
Write down the names of all of the characters of "The Lorax". These include the Lorax, the Once-ler, the Truffula Trees, the Humming Fish, the Swomee Swans and the Brown Bar-Ba-Loots.
Read "The Lorax;" as you read, write down beside each character, what creatures they are connected to. For example, the Brown Bar-Ba-Loots eat the fruit from the Truffula Trees. If you are approaching the food web more abstractly, you could write down that the Once-ler is consuming the truffula trees to make thneeds, and the factory is creating gluppity-glup, which poisons the water and affects the Humming Fish.
Review your list and make sure each animal, human and plant is connected to at least one creature. The book does not describe all of the necessary food chains for an ecosystem. If you are approaching the food web from a traditional angle and focusing on diet, you will need to create imaginary characters. For example, imagine what the Lorax eats, what eats the humming fish, what else might eat the Truffula fruit and what do the Swomee Swans eat.
Place one of your creatures near the bottom of a blank page. It is generally easiest to start with a producer, which in this case are the truffula trees.
Drawn an arrow from the truffula trees and at the pointy end of the arrow, write down a creature that consumes the truffula trees, either from the story or your imagination.
Continue in this way until every character and creature are on the page, remembering that arrows point towards the consumers/predators.