You will need...
...and a sewing machine.
NB: Negligees with narrow or spaghetti straps won't work - make sure yours has built-up straps (at least 1" wide) and is at least knee-length, as you'll have to factor in seam allowance.
ModerateIf the thought of a pattern and sewing machine fills you with dread, rest assured, it's not as hard as it looks but I don't want to shoot myself in the foot by promising you a walk-over.
Hip hip gilet...
On one side, prod along the front neckline - or back neckline, if it's lower, but that's rarely the case with wide-strapped negligees - with a pin in a dotted line.
Remove the negligee, trace along the dotted line on one side so that your outline is asymmetrical. Cut it out and cut along the middle, so that it is separated into two halves. These should look something like this:
Slip stitch the edges together, stitching inside the fold of the lining so that the stitches are invisible.
Remember to slip stitch the armholes in place. If the negligee you're using doesn't have sleeves, the edges should already be finished, meaning that no folding is required and they're easier to stitch.
Remove the pins and you should be left with something resembling this: