Objective: Select the right stitch pattern for the job and apply reliable habits so your seams look professional enough to sell.
The MC6650 gives you 170+ stitches, but for a home business you'll use only a handful constantly. The three workhorses are: the straight stitch (pattern 01) for joining seams and topstitching; the zigzag (pattern 03) for finishing raw edges so they don't fray; and a stretch/overcast stitch (patterns around 05-10, often shown with a wavy or ladder icon) for knits and for cleanly bound edges. A good default straight stitch length is about 2.5mm; shorten to ~1.5mm for strong seams on bags, lengthen to 3.5-4mm for basting or gathering. Best practices that separate a hobby seam from a sellable one: (1) Always sew a test swatch on a scrap of the same fabric and layers before touching your real piece—check that the stitch sits flat with no loops on top or bottom. (2) Balance tension: top thread showing on the back means top tension is too loose (or threading is off); bobbin thread pulling to the top means top tension is too tight. The MC6650's auto-tension is a starting point, not gospel. (3) Backstitch (the reverse button) 3-4 stitches at the start and end of every seam to lock it; skip this only when you'll cross the seam later. (4) Use the right needle: universal for wovens, ballpoint/jersey for knits—a wrong needle causes skipped stitches. (5) Keep a consistent seam allowance by lining the fabric edge against an engraved guide line on the needle plate (the 3/8" and 5/8" lines are standard). Consistency, not speed, is what customers pay for.
Example: You're making a cotton zip pouch to sell. On a double-layer scrap you select straight stitch 01, set length 2.0mm, and sew a test line—it lies flat with no loose loops, so tension is good. You finish each raw edge of the two main panels with zigzag stitch 03 (width ~3.5mm, length 2.0mm) to stop fraying. Then you place right sides together, line the fabric edge on the 3/8" needle-plate guide, backstitch at the start, sew the seam at a steady pace, and backstitch at the end. The result is a straight, locked, fray-free seam that holds up to daily use.
Try it: Take two scraps of a woven fabric. First sew a straight-stitch seam at 2.5mm, 3/8" allowance, with backstitching at both ends. Then zigzag-finish both outer raw edges. Flip the piece over and inspect: the top and bottom threads should interlock inside the fabric with no loops on either side. If you see loops on the back, adjust top tension up slightly; if the bobbin thread shows on top, lower top tension. Re-sew until one swatch passes.