This is the shirt I purchased when I was heavier:
I was able to slim the sides.
Unfortunately, fixing the shoulders was a train-wreck (now I've learned the hard way: I need to take-in the shoulders)
When I tried to sew along the thread seams, it was crooked and I couldn't put on the shirt. However, when I tried removing the threads, it made a nice hole in the shoulder area.
How to darn this hole so that it is not noticeable?
It's unlikely that you'll be able to disguise the holes in any way that's not noticeable; instead, I would recommend looking into the Japanese process "sashiko," a mending process made deliberately visible. Based on your other questions, you are quite new to sewing and embroidery, so you may wish to start with a more simple pattern like the one shown on the bottom; with practice, a wide range of more complex patterns exist that you can use in the future.
A wide range of online tutorials exist to get you started, frequently focusing on a diamond or square pattern similar to the bottom example; the repair will be solid and should help prevent further damage to the area, and will make your shirt uniquely your own.
You're asking a lot to repair that so it isn't noticeable (although if time or cost is no object it could probably be done; I've seen damaged art canvases invisibly repaired with a microscope, surgical tools, and days of work).
A more practical strategy might be to hide it in plain sight. Incorporate the repair into something like embroidery. Maybe do something matching on both sides so it looks like it was the original design.
Another idea would be to intentionally fray both sides the same way jeans are done.
A less noticable way to repair such a tear is to patch it from behind.
Cut a patch of matching fabric that covers the whole gap plus one inch in every dimension. Position the patch behind the tear and hand-sew a few stitches to keep it in place and keep the edges of the tear closed. You should use a contrasting thread color because you need to remove the stitches later.
Then choose which stitch and yarn color you want to decorate your shirt with on your sewing machine. Sew a wild criss-cross pattern all over the tear and make sure the edges of the stitches overlap the edge of the patch below (to keep it from fraying). I suggest going for a chaotic pattern, because it's very hard to sew a perfectly geometrical criss-cross with a sewing machine and any slight deviation will look odd.
At the end, cut and remove the contrasting basting thread.
The advantages of this method are:
You're slightly lucky it's torn in a fashionable way, so what I would do (and did with one of my jackets) is to use small strips (scraps, if you have them) of something like leather, faux leather, corduroy, velvet, basically any high fashion fabric.
If you're female, laces would also work. Just lace the gap from the shoulder seam all the way through the side seam only on that particular side (if your mind is able to allow asymmetry, which a matter of fact looks awesome); if not, just repeat it on the other side.
If you're using fabric instead of laces, cut a straight rectangular strip (something similar to what is often sewn on the back of jeans near the loops in the belt line, but a little less wide) and then just sew it over the torn area (yes, it's a patch, but that's a style nowadays); like above, if you want you can do it on both sides. If you want, you can also go a bit extreme! Have a fabric of that exact length of that portion of the jacket (from collar to shoulder to collar bone) and just sew it all over the whole piece. (I'm always thinking leather or leather-y texture like faux leather, but it's also fine if it is any other fabric.) If you don't want extreme but advanced and complicated, you could get some extra denim from a pair of jeans or old clothes you no longer wear, make some mini pockets out of it, and sew it on both sides to hide the torn side and the patch.
I'll be honest, there's many ways of fixing it. You just have to think a bit creatively and out of the box.
Make a basic repair of the area. And then apply on top of it a bright smiley. Or a skeleton. Or a climbing Santa.
