Objective: Construct the main body of your capstone item by joining prepared sub-assemblies into one stable, squared-up core, using the MC6650's seam guides, needle-down, and consistent seam allowances.
The 'core' is the central structure everything else attaches to later: the joined body panels of a tote, the shell of a zip pouch, or the front-plus-back of an apron. The goal of this stage is not finishing details but getting a square, true, dimensionally accurate base. Three MC6650 features carry this stage. First, use the seam allowance guide lines on the needle plate (align your fabric edge to the 1/4" or 3/8" line your pattern calls for) so every seam matches without measuring each one. Second, turn on needle-down (the button near the LCD) so the machine stops with the needle in the fabric; this lets you pivot at corners without shifting and keeps panels from creeping. Third, use a consistent stitch length (2.2-2.5mm for most cottons) and backtack at the start and end of each structural seam using the lock-stitch or reverse button. Construction order matters: join smaller sub-assemblies into larger ones, press each seam open or to one side immediately after sewing (press, don't iron-drag), and only then join those larger units. Pressing between steps is what keeps the core flat and square; skipping it is the number-one cause of twisted, puckered bodies. Check squareness as you go by laying the piece flat and confirming opposite edges are equal and corners meet at right angles.
Example: Making a lined tote core: you have two outer panels and two lining panels already interfaced. Set the needle-plate guide to 3/8". Place the two outer panels right sides together, align the raw edge to the 3/8" line, backtack, sew down one side, pivot at the corner with needle-down, sew the bottom, pivot, sew the second side. Press both side seams and the bottom seam open. Repeat identically for the lining, but leave a 4" turning gap in the lining bottom seam. You now have two boxed 'cores'—outer and lining—ready to be joined at the top in the next lesson. Laid flat, each should measure the same width top and bottom with square corners.
Try it: Take the prepared sub-assemblies for YOUR capstone item and build its core only (no handles, zippers, straps, or closures yet). Set the needle-plate guide to your pattern's seam allowance, enable needle-down, and backtack every structural seam. Press each seam the moment you finish it. When done, lay the core flat and measure opposite edges: record the two measurements and note whether corners sit square. Photograph the flat core for your project log.