Very easy to do if you don't have a sewing pattern. Purchase cheap pajama bottoms from a thrift store, they can even have holes and tears. Cut bottom apart at seams and make your own sewing pattern. Make sure that you add extra material at the top for the elastic waistband.
I've made my patterns last much longer by pinning and cutting out all my tissue pattern pieces on interfacing, then transfer all information with a permanent sharpie pen.