I'm going to make a disclaimer of sorts now, and say I have no idea how robust this design is – my sweater says hand wash only so that is what I will do – but if you have a sweater that is on its way to the sweater cemetery then you can't possibly have anything to lose.
I found this particular beauty for $4 in an east village thrift store, so I didn't have an enormous amount at stake. I'm not suggesting they all need to be quite this garish. You need to start with a sweater that fits comfortably around your hips. It also needs to be long enough from underarms to hem for you to be comfortable wearing it as a skirt. You can cut it shorter but not longer. You will also need a length of wide elastic, 1-2" shorter than your waist measurement.
Take the length of elastic you cut and sew a seam with the right sides together, to make what will become your waistband.
Open out the band you just made, and use a zigzag stitch to secure the raw edges. This will avoid any scratchiness on the waistband.
With the sweater the right side out and the waistband the wrong side out (so right sides facing), insert the bulk of the sweater inside the waistband, up under the arms. You will be sewing along the upper edge of the elastic.
Stretch out the elastic and pin in place at intervals around the sweater so that the stretch is even.
Zigzag stitch around the waistband, stretching it out as you go to ensure it is spread evenly around the sweater. If you are very brave and experienced you can cut the skirt first, but do so at you peril. I was such a scaredy puss that I zigzagged twice around the waistband before daring to cut.
When you are feeling secure enough in your stitching, you can cut the top part of the sweater off, following the line of the elastic. Rest assured that in the fullness of time I will think of something worthwhile to do with the upper body of your sweater.
Turn it back on itself and, voila, you have a skirt. Send me pictures of your own sweater-skirts and I'll stick them up here. Enjoy! xx