Per your requests, below are the steps I took to completely re-size a long-sleeve blouse. I'm sure it's not the professional way to do it (readers - please share any tips or corrections), but I just followed logical guesses. The text in red indicates when to actually sew.
1. Find a basic blouse that already fits you the way you want the new one to fit. This can be your "pattern." Turn the too-big blouse inside-out, and lay the "pattern" blouse on top of it, aligning the shoulders.
2. Using fabric chalk (you can buy these at sewing/craft stores or on eBay), mark how much to take in on each side of the torso. Start at the bottom of the armpit and go all the way down to the hem. Repeat on both sides and measure to make sure both sides are equal. Stitch along both sides of the torso, exactly where marked.
At this point, turn the too-big blouse rightside-out and try it on (yes, the shoulders and sleeves will be awkward since only the torso has been taken in). Check if the torso width is per your liking and also check to make sure the armhole width isn't too tight. Undo the stitches and re-sew, if adjustments are necessary.
3. Turn the blouse inside-out again. Use chalk and mark the new shoulder width on the too-big blouse. Trace the curve of the new armhole from the top (where the two black arrows are pointing) all the way down to the bottom of the new armhole, which is where we started marking the dotted line in step #2. Note: the armhole curvature on the front and back are not the same. Using chalk, try to mirror the curve on the "pattern" shirt to the best of your ability.
4. Using tiny scissors, snip the original stitching that attached the sleeves to the torso. Immediately mark both sleeves with chalk (the shirt should still be inside-out) to make sure we re-attach the proper sleeve to the proper side. I use numbers to make it extra clear.
5. Next, trim the armholes down to the new curved holes that we drew in step # 3. When cutting off excess fabric, make sure to leave a 1 cm margin outside of the line you traced. In this step, also make sure to trim ONLY the torso piece.
6. Now move onto trimming the sleeves. Compare the removed sleeve against the "pattern" blouse sleeve and mark the new length. Keeping the original curvature of the sleeve, trim the length down, making sure to leave a 1 cm margin again outside of the chalk line.
**If you have a blouse that fits perfectly in the shoulders/torso but the sleeves are too long, one option is to shorten the sleeves from the shoulders. I know some tailors do this for sleeves that have cuffs or other details at the hand opening that are hard to alter.**
7. Re-position the trimmed sleeve against the trimmed armhole and tack in place using a pin at the top of the shoulder seam. Make sure that the numbers we noted in step #4 match up. At this point, the entire shirt should still be facing inside-out.
8. From the armhole, reach your arm through the sleeve piece only, and turn it rightside-out, but while keeping it in place. This part is a little tricky to describe. So, if we are looking at both pieces on the floor in step 7, reach your right arm into the sleeve, grab the sleeve cuff, and pull it through back out towards the right. This entire time, the small pin at the top of the shoulder should still be in place.
9. Place the sleeve (now rightside-up) into the armhole, making sure the edges of the armhole is aligned with the sleeve hole. The torso should now still be inside out. and the pin should still be in place.
10. Holding the sleeve in place with the armhole, stitch along the edge of the armhole to attach both pieces. Per a tip from sewing expert Sew Petite Gal, start at the top of the shoulder (where our little pin is) and stitch down one side, and then go back up to the top of the shoulder and stitch down the other side. This way, if your sleeve piece is not a perfectly-cut fit for the armhole, then any mismatch will occur at the armpit and be more easily concealed.
11. After the sleeve gets attached, turn the entire blouse to the right side and try it on, making sure that no stitching was accidentally done on the visible outside. All stitching should be done on the inside-facing side.
If the armhole is comfortable and the sleeve length fits right, turn the entire blouse inside out again to make final adjustments. If the sleeves are too wide, trace a new sleeve width using the "pattern." Stitch along the new sleeve width, continuing your stitch to the armpit, and down the upper torso slightly just the clean up that junction. Fade your stitch line in with the original torso stitches we did in step #2.
You're done! Turn the entire blouse rightside-out again and try it on. If all is well, iron along all of the new stitches we made. If you have a serger machine, lock the edges of the raw seams to prevent fraying. If you don't, I use the zig zag stitching on my basic stitch machine to do a cursory locking of the raw edges.
12. Optional: The last thing I did was remove a frilly edge running along the front buttons. I thought the blouse would look less childish and more classic without this detail.
For those who are interested in learning how to sew, I highly recommend
taking a beginner's course and starting with an affordable machine like this Brother Basic Stitch one.
I took a class as a teen in China, where I learned the basics of stitching
and using a sewing machine. If anyone has recommendations for classes in
the U.S. or tips on how to begin, please share in the comments!