If the top of your sleeve pattern is an upside-down U, it's fairly easy. If it's an S-shape curve, it's a bit more tricky for beginners. The puff is formed by making the sleeve part bigger than the armhole in the body fabric. Anyway, assuming an U, just expand the U in size towards the top and "upper sides". Make sure it forms a smooth curve all the way. Sew a quick "in and out" seam by hand, and let both ends of the thread hang loose (don't knot it or cut the thread off close to the end. Pull the two thread ends to gather the fabric. Pin to hell and back to the "body fabric". You can sew the seam straight on machine, but for safety's sake stitch it quickly by hand first and try it on. The puff may be too big, too small or start too soon, in which case you just have to re-do it until it fits. If you use a different colour thread, it's easier to unpick it after you've sewn the proper seam.
If this is unclear, I'll try to explain it better.
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Thank you!
If the top of your sleeve pattern is an upside-down U, it's fairly easy. If it's an S-shape curve, it's a bit more tricky for beginners. The puff is formed by making the sleeve part bigger than the armhole in the body fabric. Anyway, assuming an U, just expand the U in size towards the top and "upper sides". Make sure it forms a smooth curve all the way. Sew a quick "in and out" seam by hand, and let both ends of the thread hang loose (don't knot it or cut the thread off close to the end. Pull the two thread ends to gather the fabric. Pin to hell and back to the "body fabric". You can sew the seam straight on machine, but for safety's sake stitch it quickly by hand first and try it on. The puff may be too big, too small or start too soon, in which case you just have to re-do it until it fits. If you use a different colour thread, it's easier to unpick it after you've sewn the proper seam.
If this is unclear, I'll try to explain it better.