This pattern is very simple indeed.

To make up the shirt proceed as follows:-

1. Hem around neck slit and opening.
2. Stitch the gussets to one edge of the sleeve.
3. Stitch the underarm seam incorporating the gusset.
4. Sew up the side seams leaving enough space to fit the arms in.
5. Stitch sleeves to body.
6. Hem the bottom edge and wrists.