Even with sleeve pinned thoroughly, it’s easy to catch and inadvertently sew other parts of the dress. Take special care, work slowly, and manage the dress fabric. Use your fingers to feel and smooth out wrinkles as your sew. If your sewing machine has a free arm, this feature might make sewing a smaller opening like a sleeve easier, check your machine’s manual.
Another option is to baste along the top of the sleeve cap. Switch to the largest stitch setting and sew, using a 1/4 inch (6 mm) seam allowance. DO NOT backstitch at the beginning or end. Leave long thread tails and gently pull to create a slight gather. Place sleeve inside the dress. Match the seams and pin sleeve to dress body as above.