Objective: Assemble the structural body of your signature product with reinforced, square seams so it is ready for finishing.
The core is the load-bearing skeleton of your product: the main panel seams that give it shape and must survive repeated customer use. On the MC6650, work in a logical order so seams you sew first stay accessible. General rule: join small subassemblies first, then combine them into the whole. Press every seam immediately after sewing (press, don't iron-drag) because unpressed seams compound into misaligned panels. For straight structural seams use stitch 1 (straight) at 2.4-2.6mm length and a 3/8 in or 1/2 in seam allowance guided by your needle-plate line. At the start and end of every structural seam use the auto lock-stitch button or reverse 3-4 stitches to prevent unraveling. At high-stress points — strap bases, corners, pocket mouths, zipper ends — reinforce with a bar tack (stitch set to narrow zigzag, length near 0.4mm, or the machine's built-in bar-tack) or sew a small box-with-X. Keep seam allowances consistent; the MC6650's accurate feed only helps if your guide line never changes. Check that panels nest right-sides-together and notches match before each seam, and use the free-arm when sewing tube-shaped sections like cuffs or bag bodies.
Example: Say your signature item is a zippered pouch. Core order: (1) sew the zipper between the two top panel edges, locking both ends; (2) press the fabric away from the zipper and topstitch 1/8 in along each side; (3) open the zipper halfway (so you can turn it later), place the two body panels right-sides-together, and sew around all four sides at 3/8 in with a locked start and finish; (4) bar-tack across each zipper end where it meets the side seam, since that is the highest-stress point; (5) press the side seams open. The pouch is now a complete turned-ready core before you add lining or a wrist strap.
Try it: Identify the three to five structural seams of your capstone item and write them in assembly order. Then sew the first two of those seams on the MC6650 at a consistent seam allowance, using the lock-stitch at both ends, and add one bar tack at your item's single highest-stress point. Press each seam as you go, then lay the piece flat and measure the finished seam allowance in two places to confirm it is within 1/16 in of your target.