Find a container
Any ventilated container large enough for a few stems of greenery will do.
A jam-jar or margarine tub is ideal, or even a cardboard box with two sides replaced with muslin.
Alternatively, use a muslin or net bag to enclose a caterpillar on an area of a plant.
This keeps it contained, but allows it to feed naturally.
Create a home
Line your container with kitchen towel to absorb moisture, and introduce a few stems from the plant on which you found the larva.
Replace these at least every couple of days.
Add a layer of soil in case your species pupates by burrowing, and a twig for the adult to climb and expand its wings.
Place your caterpillar inside then put the container in a cool spot out of direct sunlight.
The larva will feed until fully grown, then will pupate.
Keep it clean
Remove droppings (‘frass’) and old, dead leaves.
It will take at least a few months for the adult to emerge – release it where you found the larva.