There are two methods for sewing sleeves. Neither method is particularly difficult, although sewing sleeves in flat tends to be easier. Sewing patterns for set-in sleeves tend to have a higher sleeve cap, while patterns for sleeves that are sewn in flat, such as robes, have a wider or somewhat broader sleeve cap.
Sewing set-in sleeves is often more intimidating to sewing enthusiasts. Most blouses, jackets and coats have set-in sleeves. To sew set-in sleeves, the shoulder seams and the side seams of the bodice are already sewn and pressed open. The armhole that is created in the bodice is called the armscye.
The next step after sewing the bodice shoulder and side seams is to prepare the sleeve. Most set-in sleeves are supposed to be smooth at the sleeve cap when the garment is being worn, but this can be a bit tricky to achieve during the sewing process because the sleeve cap must be eased in order to fit into the armscye properly. Personally, I like to set my stitch length on about five and sew these easing stitches while the sleeve is still flat. Most sewing patterns will instruct you to sew at 5/8 inch and again at 1/2 inch, which is easier when you’re first learning how to sew, but I like to sew at 3/8 inch and just a little bit less than 3/8 inches instead because when I sew the seam to join the sleeve to the garment at a standard 5/8-inch seam allowance, the easing stitches don't show, although it is easy enough to pull them out if they should show after sewing the sleeve into the bodice.
After you sew the stitches on the sleeve cap, then you should sew and press open the sleeve seam. When I sew set-in sleeves, I prefer to finish the raw edges of the seam prior to sewing construction, sew the seam and then press it open. This creates less bulk, especially in the underarm area where the seams intersect. Now you are ready to insert the sleeve into the armscye. Remember that the right sides of the fabric will be placed together, so the bodice will be turned so that the wrong side of the fabric is facing you. Match the sleeve seam and the bodice side seam as well as the notches. A double notch indicates the back of the sleeve, and a single notch indicates the front of the sleeve. Pull the stitches on the sleeve cap so that the sleeve fits into the armscye. Then it is time to sew the sleeve to the bodice.
Using the free arm of your sewing machine when sewing set-in sleeves makes it easier to sew. Some seamstresses place the bodice next to the feed dogs of the sewing machine and sew the sleeve in that way, but I prefer to place the pinned sleeve on the free arm of the sewing machine (with the sleeve next to the feed dogs) and sew around the armscye, slightly overlapping where I started sewing, which is usually at the seam. I like to finish the seam that is created when the set-in sleeve is joined to the bodice. I usually serge finish this seam. If the set-in sleeve has been sewn in correctly, there should be no puckers on the sleeve cap when the garment is turned right-side out.
Another method for sewing in sleeves is to sew them in flat. You sew and press open the shoulder seams on the bodice, but leave the side seams unsewn. Although many sewing patterns don't instruct you to ease the sleeve cap to fit, I often have to slightly ease the sleeve cap for it to fit correctly. Place the sleeve on the bodice with the right sides of the fabric together and match the notches on the open (because the side seams are not sewn yet) armscye, pin the sleeve and then use your sewing machine to sew the sleeve cap to the bodice. This seam will be slightly curved but is quite easy to sew. If the sleeve has been sewn correctly using this method, the sleeve cap will be smooth with no puckers.
The next step is to sew the bodice side seam and sleeve seam as one seam. When you come to the area under the arm, make sure you match the seam lines (where the sleeve was joined to the bodice). Sometimes you need to clip the seam under the arm because this allows the seams to be pressed open more easily.
When sewing sleeves, remember that sleeve caps should be eased in order for the sleeve to fit into the armscye properly. Most sewing patterns have a dot on the pattern, and this dot should be marked on the fabric and placed on the shoulder seam, which makes it easier to fit the sleeve into the armscye. Matching the notches is also important. Double notches indicate the back of the sleeve, while single notches indicate the front of the sleeve, so cutting the notches is always important when cutting out sleeve patterns. When easing the sleeve cap, the threads are slightly pulled in order for the sleeve to fit into the armscye. These easing stitches are not gathering stitches, which are pulled much tighter.
The key to sewing sleeves is to not rush the process. Learning how to sew sleeves may sound challenging at first, as you will see, it really isn't too difficult.