Objective: Sew a controlled straight seam with a 5/8" allowance, lock both ends with a backstitch, and pivot cleanly at a 90-degree corner.
Every product you'll sell starts with three reliable skills: a straight seam, a secured stitch, and a clean pivot. On the MC6650, select straight stitch (U1 pattern 1) with the needle in center position and set length to about 2.5mm for general seaming. Seam allowance is the distance from the needle to the raw edge of your fabric. Line the fabric's right edge against the 5/8" (16mm) guide line engraved on the needle plate and keep it there as you feed. Don't push or pull the fabric; let the feed dogs pull it through while your hands only guide. To stop a seam from pulling apart, backstitch: at the start, sew 3-4 stitches forward, press the reverse button (held on the MC6650 for as long as you want reverse), sew back over those stitches, release, then continue forward. Repeat the reverse at the very end. For corners, stop with the needle down in the fabric (press the needle up/down button so the needle stays buried), lift the presser foot, rotate the fabric around the needle to the new direction, lower the foot, and keep sewing. The buried needle acts as a pivot pin so the corner stays sharp and aligned.
Example: Take two 6"x6" cotton squares, right sides together. Place the right edge on the 5/8" line. Lower the foot. Backstitch 3 stitches, sew forward down the edge at a steady slow speed keeping the edge on the line. About 5/8" before the bottom corner, press needle-down, lift the foot, rotate the fabric 90 degrees, lower the foot, and continue along the next edge. At the final corner, sew to the end and backstitch 3 stitches to lock. You now have an L-shaped seam with square corners and secured ends.
Try it: Cut two rectangles of scrap cotton about 5"x7". With right sides together, sew around three sides using a 5/8" seam allowance: backstitch at the start, pivot at both corners using needle-down, and backstitch at the end. Then examine your work: the seam line should be straight and parallel to the edge, corners square, and the start/end stitches doubled.