It's super easy and even if you don't have a super industrial machine you can stitch one up in under 30 mins.
1. First, using fabric scissors cut the sleeves off following the seams.
2. Turn the shirt inside out. Use a top that fits you well as a template. Center the template top over the shirt and use dressmakers chalk or regular chalk to mark the side seams and shoulder straps. I didn't worry too much on being perfect. I guesstimated and aimed for symmetry from side to side.
3. Cut the excess from the shoulders and pins the sides.
4. Sew along your chalk marks.
5. Cut the neckline. I just followed the seam. You can always cut more, but first try your shirt on. Adjust and cut more if needed/desired to even out the straps or neckline.
6. Stretch the arm holes and neck line a bit till they curl in. Best thing about jersey- no need to sew raw edges!