Open back adaptive slacks (from slacks with elastic waist bands)

"To adapt slacks with an elastic waist band is incredibly simple and only requires a pair of scissors. First of all cut the back seam open (or just cut it out) from an inch or two below the waist band to an inch to an inch and a half before the crotch seam. If simply opening the back doesn’t keep the edges out of the line of fire, cut a few inches straight out to the side seams on each side of the opening. Make the cuts at about hip level. Don’t cut more than halfway to the side seam or you may have a gap showing bare skin. If the fabric is something that will unravel, use a sewing machine to zigzag stitch next to the cut edges to prevent unraveling. If you make the hip level side cuts, a couple rows of stitching across the end of the cut will help keep it from ripping out farther", she explained.

Adapted from: http://www.alsfrombothsides.org