Once you obtain a live female butterfly, it is best to place her in some sort of container and then place her in a cooler where she can calm down if you plan on spending more time in the field.  Some place females of larger butterflies i.e, Papilio, Limenitis,  Speyeria, or other brushfoot butterflies in a glassine envelope while remaining in the field.  This practice is NOT advisable for butterflies that are smaller and/or more fragile which includes most pierids, lycaenids, and some skippers.

Live female butterflies can live for days in captivity and need to be fed regularly. The key to do this is to mix honey with water 1:10 and place on a paper towel or cotton ball. Females of many species of butterflies will automatically uncoil their proboscis to nectar once their feet touch moisture if they are thirsty. Other species such as swallowtails, some satyrids, and the pine white, will require you to uncoil their proboscis to coax them to nectar. The images below demonstrate.