Some rules I will always follow to the T though. Using turn signals for every change in direction while driving, always brush my teeth before bed, don't feed the cats too much no matter how they beg, and always baste sleeve caps before sewing. ALWAYS.
So when you end up with a fold like above, it's not a big deal. Just rip the seam about an inch on either side and baste it in again.
You end up with a ruffly thing like this which you then massage into place. Literally massage with your fingers until it's all smoothed out.
And nice and smooth on the inside too.
But Mandykatt, how do you put together the two pieces in the first place? The sleeve has way more fabric than the armhole does! True. And it's a good thing it does or you'd have to walk with your arms straight out the sides because there'd be no ease. I use this method for pinning a curved piece to a straight piece too, like the bodice front with princess seams. My teacher showed me this in high school, so I don't actually know if there is a better method, but this produces a nice result so I keep using it. If anyone knows of a more efficient method for easing in sleeves, let me know please, I'm always eager to learn new techniques.
Get out your pin cushion, you're going to need lots of pins. Start by lining up the notches on either side of the sleeve with the ones on the bodice. These are usually placed just below the area to be eased in (sleeve cap). Right sides together (turn your bodice inside out and the sleeve inside out, and slip the sleeve inside the bodice) and matching front and back notches (back has double notch, front has single for most patterns, read your instructions / photo match your pieces) pin the sleeve to the bodice at the notches. Then pinch the two pins together and find the middle of the sleeve cap on both pieces. Mark those points with pins, then match them and pin together.
Keep folding in half between two pins, finding the middles and pinning together. The steeper the curve the closer the pins should be.
Try to stay within the seam allowance to keep the garment maneuverable under the sewing needle.
Raw edges should also line up the whole way around. Pin the bottom of the sleeve to the bottom of the armhole without easing.
Handle carefully or that's going to hurt.
Baste in slowly, removing pins as they reach the presser foot.
Yay for smooth sleeves! I know some people are intimidated by sewing in sleeves, but it's really not that big a thing. Baste first, use lots of pins, and go slow. You'll be pro in no time!