To start out I laid the sweater out flat and beginning at each of the bottom corners I cut diagonally up the top.
The front and the back were now connected by just a few inches of sewing on each side of the collar. Using a seam ripper I separated them and positioned the pieces so that the good sides were together and sewed along the two sides, stopping on each side 3" from the top.
By cutting off that excess fabric and then opening the seam on one of the sleeves I was able to quickly come up with the decent makings of a bodice.
Because my machine doesn't love to sew sweatery stuff, I added one extra step here. Before finishing off the sides I laid down some double stick tape ¼” away from the edge and then gently turned the area to be sewn over it to make a nice clean line. That little bit of adhesion was enough to stabilized the edges and prevent unfortunate bunching once I got it on the sewing machine.
To create the straps I cut the twill tape in half, giving me two 11” pieces, put them in position at the top corners of the bodice and sewed them into place.
The very last step was attaching the bottom of the bodice to the front top edge of the skirt, by sewing between the two pieces of elastic.