Objective: Sew a secure, straight seam with a 5/8" seam allowance and pivot cleanly at a corner using the MC6650's core controls.
A clean seam is the foundation of every sellable item. On the Janome MC6650, straight stitch is pattern #1; set stitch length to about 2.4mm for standard seams. The seam allowance is the distance between the needle and the fabric edge, not the needle and the foot. The 5/8" (1.6cm) guideline is the standard for most garment and home-goods patterns. Use the engraved lines on the needle plate: line your fabric's raw edge up with the 5/8" mark and keep it there as you sew, watching the edge against the guide rather than watching the needle. Feed the fabric gently with your hands guiding, never pulling; the machine's feed dogs move it for you. Pulling distorts stitches and can break needles. To lock a seam so it won't unravel, backstitch: sew 3-4 stitches forward, hold the reverse button to sew back over them, then release and continue forward. Repeat at the end. To turn a corner, sew up to the point where the seam allowance from the next edge intersects, 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, lower the foot, and continue. The MC6650 lets you set the needle to stop in the down position automatically, which makes pivoting effortless and keeps your line unbroken.
Example: Take a scrap rectangle. Select straight stitch #1, length 2.4. Align the right raw edge with the 5/8" needle-plate line. Lower the foot. Hold reverse and sew 4 stitches back, release, then sew forward along the 5/8" line. About 2cm before the corner, press needle-down so it stops buried; lift the foot, rotate the fabric 90 degrees, lower the foot, and sew the next edge. At the finish, backstitch 4 stitches. The result is a square-cornered seam that holds under tension.
Try it: Cut two 6"x6" fabric squares. Place them right sides together and sew around three sides with a 5/8" seam allowance, backstitching at the start and end of each continuous run and pivoting at the two corners using needle-down. Measure your seam allowance at several points with a ruler and check that your corners form clean right angles.