I've been meaning to do this upcycle forever. I bought this extra-large long sleeve tee at the thrift store a couple of months ago and planned to use a pattern to make it fit me. I love reusing hems any time I can. It makes sewing so fast! The original tee is on the left and the finished/altered tee is on the right.
A tee two sizes larger than your regular size
A tried-and-true tee pattern (I used the Union St. Tee* with these alterations.)
1. Cut open the tee along the side seams.
2. Carefully lay your pattern on the front so that the pattern is laying only on fabric. I lined up the edges of the neckband with the edges of the neck on the pattern. The bottom went past the hem slightly but there is hem allowance in the pattern to take into consideration.
3. Do the same to the back and line up the edges of the neckband with the edges of the neck on the pattern so that the front and back neckband will line up.
4. Lay the sleeves on top of each other and facing opposite ways. Cut out the sleeves. I didn't have the long sleeve pattern piece printed out so I made do with the short sleeve pattern piece.
5. Construct the tee as the pattern directs but the hems and neckband will already be in place. I like to sew to the edge and backstitch and then serge the edges. Then I tuck the serging tails under the seam allowance and tack it with my sewing machine. I usually do a diagonal from the seam and hem edge away from the seam at an angle. This insures that the serging tails is stuck under the seam allowance. Then I trim any strings hanging out. Here is a view of the outside and inside of my tacking.