With RST, pin sides and sleeve, matching underarm seams. Serge/stitch underarm and sides in one continuous seam, stopping at notch. NB: If you're using a serger, serge off the edge to stop at the notch.
To help prevent neckline stretching out of shape, you may like to staystitch the neck at this point. Simply stitch at 8mm(1/3") working from shoulder to CF (centre front) on both sides of Front, then similarly from shoulder to CB (centre back).