the first thing i normally do is take out the side seems and remove the sleeves. with the pink shirt, i didn't need to remove the sleeves, with the black shirt, i removed the sleeves and* i also removed the upper-back seem.
next, i cut my fabric down to the size i wanted it (making sure to leave extra room for the darts).
here are where i put my darts:
the orange darts i did on both shirts. the green, i only did on the black shirt.
after the darts where in place, i made my new side seems (and also re-sew the upper back seem of the black shirt).
the reason i didn't take the sleeves off the pink shirt was that they were already puffy. they were also very wide, so when i sowed up the side, i continued that seem to make the sleeves narrower.
with the black shirt, i removed the sleeves. to make them puffy, i stitched the shoulder area and then gathered my stitch to make a bunch. after that, i simply refitted the sleeves back into their new sleeve holes.
to make the sleeves shorter i just cut them off, and then either re-put the old cuffs back on, or just make new ones. i also gather the seem a little around the bottom of the sleeves before attaching the cuffs for extra poof.