I finally figured out how to alter all those big men's tee shirts I've bought. I have to buy a 5X to get them to fit my rack of doom and to fit around my hips, but then the shoulders are waaaaaaay too wide - the shoulder seam comes halfway down my upper arm and the sleeve then hits me below my elbow - not a good look, even in a tee shirt.
I took one that I didn't care about ruining, and removed the sleeves (I just cut them off very close to the seam and then trimmed the seam off the shirt). See the picture below for what the deconstructed tee looks like:
The first thing I did was measure how wide I needed the shoulder to be, from my neck to the edge of my shoulder. On me, that's approximately 5 1/2 inches. I added 1/2 inch for seam allowance, and cut from the shoulder down to where I thought the sleeve opening should end, curving it slightly to point A (where the dotted line is on the diagram of the tee). This left about 3" of the original sleeve opening that isn't needed. With the tee turned inside out, I made a very narrow dart starting at point B and ending at point A (1/2" wide at point A).
I then took the sleeve, laid it out flat and smooth (just like I cut it off the shirt, didn't open up the underarm seam, you want that left sewn up). I then cut a curved line along the seam line of the sleeve (the dotted line on the sleeve). This makes the tee sleeve just like a blouse sleeve. I then put a small notch at point C on the sleeve, to match with the shoulder seam on the tee.
It's a good idea to mark which sleeve came off which side of the tee so you know which side to sew it back to when you're done making your alteration cuts. I did this by putting a small safety pin in one shoulder of the tee and its matching sleeve (the other shoulder had no pin and neither did the sleeve, making them a pair).
By matching the notch on the sleeve top with the shoulder seam on the tee, and the underarm seam of the sleeve with the narrow dart you made in the armhole of the tee, you can then finish pinning the sleeve into the tee, easing any fullness at the top (shoulder area or cap) of the sleeve and then sewing it in place. I left the tee inside out, the sleeve was right side out, and put the sleeve inside the shirt to pin and sew it - setting a sleeve "in-the-round" is easier this way and you can put the sleeve on the bottom/tee on top which makes it easier to "ease-in" any fullness you may have in the cap of the sleeve.
This isn't a project for a novice seamstress, but I think anyone who has made a lot of blouses/tees/tops could handle this project. My first attempt involved some ripping out of seams and resewing them, but it worked, and the tee looked good when I finished it, and best of all, it fit much better (and that narrow dart under the arm doesn't show when I'm wearing the shirt either).
ETA : Sorry, I'm not much of an artist, that's the best I could come up with for what I did and how I did it.....LOL! I also don't like how the crew necks fit me, they're too tight and I'm always yanking at them, so I altered them too. I took my scissors and snipped the crew neck banding from the edge to the top-stitching (made those snips every 3/4"). I then turned the crew neck under completely, pinned it in place, and top-stitched it down with thread that matched the tee shirt. After the top-stitching was complete, I trimmed off the excess crew neck fabric close to the top-stitching. Voila! A larger neck opening that isn't tight and still looks neat and clean.