This info is from a friend who raises butterflies:

Keep it in a mesh laundry basket that stands freely (not a bag). You can find them in the laundry basket dept. You can buy mesh in the fabric dept, to put over the top to keep it closed, and use binder clips so the mesh stays on it. Butterflies feed on nectar. Soaking Red Gatorade on paper towels will work, as well as watermelon, squished apple, simple syrup, etc. They are easy to keep. You can put your finger up to it gently and it will go on it. Put the butterfly on top of the paper towel or fruit. They taste with their feet and once know it has food, will leave your finger. You can put the food onto a paper plate, and put the plate inside your habitat. They like to be in the sun, but be careful of passive solar- you don’t want to fry it. Mostly they will hang from the top of the habitat during the day. At night they go to the top and close their wings up to sleep. It’s good to keep them in a dark place at night. They are hardier than they look. Good luck.