Even really good sewers are afraid of knits. They're afraid of the knit stretching and not being able to control it. Right from the get-go, you want to build in some stabilizing. In this video, I show you how to add fusible tricot to shoulder seams. It will keep your neckline from stretching while you work.
Use a lightweight tear-away stabilizer like Totally Stable on the underside of the dart to keep the narrow fold of the dart smooth and the stitches even. This will prevent stretching and keep the end of the dart from being sucked down into the needle hole. You can change the stitch length to small stitches for the last 1/2 inch at the tip of the dart to secure the threads without the need to tie them off.
I work on a tailor's ham a lot. I press the dart flat to blend in the stitches. Then you want to press your darts, going up. It will give a more youthful appearance, as you wear it. Remember that you want to make your darts as invisible as possible; all but disappearing when you wear your garment.
Sewing with the interfacing on top will keep your fabric from stretching, buckling, and puckering as you sew. Threading your pins through before you begin will help to hold your pieces in place. After stitching, press each of the shoulder seams flat and then open, toward the back, to further set the seams.
You can use any width double needle. I prefer the 4.0mm #12/80 size. You can wind wooly nylon in the bobbin, and use black for dark colors, and beige for light. The wooly nylon in the bobbin keeps the knit from stretching in most cases. BUT…if you see your fabric stretching, use stay stitching and as you sew, apply pressure to the back of the presser foot, using one finger and an even tension.
The potential for knit sleeves growing as you work with them is very real. But little sticky dots along with fusible web (pressed in the stable vs. stretchy direction) for the entire length of the sleeve can help you avoid this problem.
