1) Establish the center front. Fold the shirt in half down the front,
matching the armholes and side seams. Iron along the center front fold
to create a press line.
2) Remove the neck ribbing, sleeve cuffs and sleeves using a seam
ripper. These will most likely be serged together, so be patient.
3) Cut the shirt body in half along the press line.
4) When putting the shirts back together (I assume they are a stretchy
material like knit) use a stretch stitch like zigzag or triple stitch.
You may find it easier to put the sleeves on to split the shirt seams
down the sides as well. It would make it easier to sew the sleeves
back on and then you could stitch the sides back. I usually start at
the sleeve opening and stitch down yo the underarm and then down the
side in one continuous seam. Use the smallest seam allowance you can
to sew the two sides of the shirt body together so that you don't
change the size of the neck opening too much.
5) When applying the neck ribbing, stretch it to fit the neck opening
while you sew. Mark the ribbing in fourths and then match these marks
to the center front, center back and shoulder seams. That way the
stretch is even across the neck opening.
I noticed that there is no sleeve ribbing on the shirt so just fold
under and hem with a stretch stitch.
If you are confused, I suggest you look at a sewing pattern for a t-
shirt. The parts should all be the same, so the directions should
work. The only difference will be that the shirt body is pieced of two
colors. Good luck.