Cut the squash from the vines. Use gardening shears to cut the stem of the squash 3 inches from the top of the squash to leave a portion of stem attached. The stem reduces the risk of rotting.
Wash the outside of the squash with a lukewarm, soapy water solution to remove dirt and reduce the risk of mold development. Rinse the soap residue away then dry with a towel.
Place the squash in a windowsill that gets heavy sunlight. Turn it halfway through the day each day to distribute the sunlight evenly, ripening the squash on all sides. Leave it there for three or four days if needed, turning it regularly, to complete the ripening process.