Objective: Select and dial in the most-used MC6650 stitch patterns and apply professional habits so your first sellable items have strong, tidy seams.
The MC6650 has 170 built-in stitches, but a home-business maker uses only a few constantly. The straight stitch (pattern 1 on the direct-select panel) joins seams and topstitches; set stitch length around 2.5 mm for general seams and 3.0-3.5 mm for topstitching on thick fabric. The zigzag (pattern 3) finishes raw edges, sews stretch fabric, and attaches elastic; a width of 3.5-5.0 mm with a length of 1.5-2.0 mm is typical. The overcast/overedge stitch (around pattern 8, used with the overedge foot M) mimics a serger to keep edges from fraying on items you sell. Three best practices govern all of them. First, always sew a test swatch on a doubled scrap of the exact fabric before touching your project; check that the thread lies flat and the fabric doesn't pucker. Second, tune tension by the balance rule: if the top thread pulls loops to the underside, lower the top tension number; if the bobbin thread shows on top, raise it. Normal auto tension on the MC6650 is around 4. Third, match thread, needle, and fabric: 50-weight all-purpose polyester with a 75/11 or 80/12 universal needle suits most cottons; use a ballpoint needle for knits. Backstitch (reverse button) 3-4 stitches at the start and end of every seam, or use the auto lock-stitch, so seams don't unravel in a customer's hands. Press each seam before crossing it with another; pressing, not sewing, is what makes a finished item look professionally made.
Example: You are making a cotton zippered pouch to sell. You load 50wt polyester thread and an 80/12 universal needle. On a doubled scrap of the pouch cotton you select straight stitch pattern 1, set length to 2.5 mm, and sew a test line: the stitches lie flat and balanced, so tension stays at auto 4. You sew the side seams, backstitching 3 stitches at each end. Then you switch to the overcast stitch with foot M and finish the two raw seam allowances so they won't fray. Finally you press the seams open before topstitching the top edge at length 3.0 mm.
Try it: Take two scraps of a medium-weight cotton. (1) Sew a straight seam at 2.5 mm length and backstitch both ends. (2) Switch to zigzag at width 4.0 mm, length 1.8 mm, and finish one raw edge. (3) Examine both the top and underside of each stitch line. If bobbin thread shows on top or loops appear underneath, adjust the top tension one number and re-test until both sides look balanced. Write down the final settings that gave you a clean result.