Use the mattress stitch - video found here
under 'Finishing'. You will have 4 sleeve seams in total. Lay the sleeves and front and back pieces down flat, with the right sides of the garment facing out. Match up the raglan seams and start seaming. I find it's best to start your seams from the neck edge and end at the underarm, that way any imperfections in the seaming will be hidden under the arm.