Begin by searching in plants and bushes during late spring or early summer. Keep looking until you find a caterpillar that you would like to keep. Do not take a caterpillar from the ground unless you are an expert. If it is on a leaf or plant stem, you will know what it eats and be able to keep it alive in captivity.
Try not to touch caterpillars, as their body oils can irritate your skin. Instead, remove the section of the plant or bush that the caterpillar is resting on. Also take other leaves from the plant to feed your caterpillar in the weeks to come.
Place the plant stem, leaves, a stick and the caterpillar in a large jar or old fish tank. The bottom of the container should be filled with a half inch of either soil or sand, because some caterpillars burrow into the ground when they pupate [become butterflies]. A branch must be placed in the container vertically in case the caterpillar hangs when it pupates.
Keep the caterpillar alive by feeding it. To do so, periodically add leaves and stems from the species of plant that the caterpillar was originally found on. To keep the plant pieces fresh, put the stems in a small jar of water and place it in the caterpillar container. To prevent the caterpillar from accidentally drowning, stuff cotton balls around the sides of the plant pieces in the water.
Place a cheesecloth or mesh layer over the container and secure it with a piece of string or rubber band.
Once the moth or butterfly emerges from the pupal form, take the container outside and release the insect where you first found it. It may need to dry its wings for a few hours before flying. Be patient and do not force it out of the container.