Objective: Turn the individual skills you've learned into reliable muscle memory by running a repeatable practice routine and learning to diagnose and fix your own stitching.
Confidence at the machine comes from repetition under light pressure, not from one perfect seam. On the Janome MC6650 you have tools that make practice easier: the needle up/down button lets you stop with the needle down to pivot cleanly, the speed-control slider lets you cap your maximum speed so the pedal can't run away from you, and the seam guide lines etched on the needle plate give you a target to steer toward. The habit to build is: eyes on the guide line and the fabric edge, not on the needle. Let the feed dogs pull the fabric; your hands only steer. Three beginner errors cause most crooked, puckered, or messy seams. First, wavy seams come from gripping or pushing the fabric, which fights the feed dogs; the fix is to guide lightly and let the machine feed. Second, uneven stitch length or looping on the underside usually means uneven speed or a tension/threading issue; slow down and re-check your top thread is seated in the take-up lever. Third, seams that drift off the guide come from watching the needle instead of the edge; move your gaze forward to the guide line. Practicing deliberately—same drill, checking results each pass—builds the steady pace that production sewing for a business will demand.
Example: Set the speed slider to about half. Cut a 6"x6" scrap of medium cotton. Line the right edge against the 5/8" guide line, lower the presser foot, and press the needle-down button so your first stitch anchors. Sew a slow straight line, keeping the fabric edge glued to the 5/8" line while looking at that line, not the needle. At the end, lift the foot and inspect: the top stitches should be even and the same length, the underside smooth with no loops. On a first pass many beginners find the line waved inward at the middle—caused by subconsciously pulling the fabric. Repeat with hands flat and relaxed, steering only, and the second seam runs noticeably straighter.
Try it: Cut six scrap squares. Sew a 5/8" seam on three of them and a 1/4" seam on the other three, using the needle plate guide lines and the speed slider set to half. After each seam, check it against the three error signs: wavy line, uneven/looping stitches, drift off the guide. Write one word next to each square noting what to fix, then sew the next one applying that fix. Keep the best 5/8" and best 1/4" sample as your reference pieces.