i am so sorry about the blurries. i didn't realise until too late (that is, both seams were done). hopefully they work as a kind of visual hint (rather than a visual guide) to help you understand my fabulous written instructions.
firstly: this is what happens when you get very frustrated, and try to use kitchener stitch to join the shoulders on a jumper where each piece is cast off. See how it looks weird and ruckered?
That is not what you want. You want delicious, smooth, invisible seams. So you pick apart the mess you have already made. You lie your two shoulder pieces facing flat, right side up, cast off edges butting up against each other. the back is the 'top' piece, and the front of your jumper is the 'bottom' piece. Take a tapestry needle threaded with yarn, and poke it through the very corner of the outer shoulder of the top piece. Leave a tail enough on the wrong side to darn in afterwards. Then, look for the first vertical row of Vs made by your wonderful stocking stitch. See the V closest to the cast-off edge in the very right-hand column of Vs? Slip your needle under that V. Like in the rather lousy picture.
Then, find the V closest to the cast-off edge in the far right hand column of Vs in the bottom piece. Slip your needle under the little bars of that V too.
Pull the thread to a nice, even tension. So it looks about the same as your actual knitting. Then, find the V in the second column from the right in the top piece. Needle under the bars, right to left. Repeat for the bottom piece.
Do it again, and again, moving along the columns. It should look pretty much like this:except not red if you aren't using red yarn. Don't fret about the colour.
When you're all finished, and all the Vs in the columns that butt up against each other have been stitched under, stick your needle through to the wrong side of the fabric. Darn in the end. And don't forget to darn in the other end too! Do it now before you forget!
Flip over the right side of your work again, and admire your beautiful seam. Even the camera likes this seam enough when it's finished to go back into focus.
Now go and have a cookie, because you, my friend, have just sewn together a beautiful shoulder seam.