Decide which ecosystem you want a diagram for -- perhaps a rain forest, a coral reef, your own garden or a lake.
Make a list of the main animals, plants and other organisms in the ecosystem. For example, your own garden has bacteria, fungi, plants, many invertebrates, small rodents, birds, small carnivores such as spiders, foxes and weasels -- and perhaps the odd visit from larger carnivores and herbivores.
Draw a box on the top left of the paper. Plants are the primary producers in most ecosystems and the food web starts off here.
Draw a second box underneath. This is for bacteria, fungi and detritivores (like earthworms), which break down organic matter. Put a box to its right for scavengers.
Add further boxes on the right of plants for, in order, herbivores, primary carnivores and secondary carnivores.
Go through your list and write the name of each animal, plant or other organism in the appropriate box. Some species belong in several boxes. Rats, for example, go in the herbivore, primary carnivore, secondary carnivore (when they eat spiders) and scavenger boxes.
Add arrows showing which organisms consume which others. Plants need arrows leading to detritivores and herbivores. Primary carnivores eat mainly herbivores and detritivores and, in turn, are eaten by secondary carnivores and, when they die, by detritivores and scavengers.