You'll need a t-shirt that fits, a set of sweater sleeves, your scissors and pins and a sewing machine or serger for this. It takes about ten minutes of your time so you'll have something to wear out on that first chilly day of the year.
Start by turning your t-shirt inside out and cutting the sleeves off carefully at the seam.
Pin your sweater sleeves inside your t-shirt and sew using a zig-zag stitch or serge. That's it! You're done!