Objective: Sew a clean, straight, locked seam at a 5/8" allowance and pivot a square corner on the Janome MC6650.
Every sellable item depends on three repeatable skills: a straight seam, a locked seam, and a clean pivot. On the MC6650, set straight stitch (pattern 00 on the direct-select panel) with a default stitch length of 2.4mm. The needle plate is etched with guide lines; the one marked 5/8" (about 1.6cm) is the standard garment and home-goods seam allowance. Align the raw edge of your fabric with that line, not the needle, and watch the edge against the line rather than watching the needle. To lock a seam you backstitch: sew 3-4 stitches forward, press and hold the reverse (reverse-stitch) button to go back over them, release, then continue forward. Repeat the back-and-forward lock at the seam's end. This keeps threads from pulling out when the item is used or washed. To pivot at a corner, sew until the needle reaches the corner point, stop with the needle DOWN in the fabric (press the needle up/down button so it stays in the fabric, or turn the handwheel toward you), lift the presser foot, rotate the fabric around the needle to the new direction, lower the foot, and keep sewing. The needle acts as a hinge so the stitching lines meet precisely. Keep a steady, moderate speed using the speed slider and let the feed dogs pull the fabric—don't push or pull it.
Try it: On two layers of scrap cotton about 8" square, draw or imagine a line 5/8" from one edge. Load straight stitch, length 2.4. Start 1/2" in, backstitch 3-4 stitches, then sew forward keeping the raw edge on the 5/8" plate line for about 4". Stop with the needle down at a corner, lift the foot, rotate 90 degrees, lower the foot, and sew another 4". Finish with a backstitch lock. Check that both seam legs stay at 5/8" and the corner is a crisp right angle.