Objective: Assemble the primary body panels of your signature product into a square, durable core, pressing and verifying each seam before moving to details.
The core is the structural skeleton of your product: the main panels joined in the order that makes later steps possible. Work from the inside out, joining the largest flat panels first so the piece stays manageable and seams stay accessible. On the MC6650, select a straight stitch (stitch #1), set length to 2.4-2.5mm for standard construction seams; drop to 1.8mm only where you need extra strength at stress points. Pin or clip panels right sides together, matching notches and corners first, then easing the rest. Sew with a steady medium speed using the MC6650's speed slider so you keep an even seam allowance against the guide lines on the needle plate. The non-negotiable rhythm is sew, press, check: press every seam flat and then to one side (or open) with your iron before crossing it with another seam, because an unpressed seam hides fullness and throws off the next join. Check squareness as you go by measuring diagonals of the assembled core; equal diagonals mean square corners. Finish raw edges as you build using the overcast stitch or a zigzag so interior seams don't fray during use and washing. Keeping this exact order documented lets you repeat it identically across many units, which is what turns a one-off into a sellable, consistent product line.
Example: Say your signature item is a lined zip pouch. Core order: (1) sew the two exterior panels right sides together along the bottom, straight stitch 2.5mm, 1/4" seam; (2) press the seam open; (3) box the bottom corners with a reinforced 1.8mm straight stitch and overcast the raw corner edge; (4) measure the opening width at top and 2" down—both read 8", confirming the body is square. You now have a finished exterior core ready for the zipper and lining in Lesson 4.
Try it: Take your two to four main body panels and assemble the core only (no zippers, straps, or linings yet). Set the MC6650 to straight stitch, length 2.5mm. Join the largest panels first, matching notches and corners. After each seam: press it, overcast or zigzag the raw edge, then sew the next seam. When the core is closed, measure both diagonals (or opposite edges) and write the two numbers down. They should match within 1/8". Keep a short written list of the exact seam order you used.