You won't need to do much once the caterpillar successfully pupates. Remove the food plant. The pupa can dry out if conditions become too dry, or become moldy if too damp. Some lepidopterists recommend removing the pupa from the caterpillar housing, but this isn't necessary if you check the jar once in a while. If the soil appears extremely dry and crumbly, a light spray with water will add a little moisture. If condensation appears on the jar, wipe it down.
Spring and most summer caterpillars may emerge as adults within a few weeks after pupating. Fall caterpillars usually overwinter in the pupal form, meaning you will have to wait until spring to see the moth or butterfly. I recommend keeping any overwintering pupae in a cool basement or unheated garage, to prevent premature emergence. You don't want a butterfly flying around your home in winter.
When the adult emerges, it will need time to dry its wings before it can fly. This may take a few hours. Once it is ready to fly, it may begin fluttering its wings rapidly, which can damage its wings if the butterfly or moth is left in the jar. Take the jar outdoors, preferably to the area where you collected the caterpillar, and release your butterfly or moth.
