The biggest complaint we get from students: "My seams are all crooked!" We have devised a foolproof method to keep your seams straight.
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1. Put a piece of masking tape on your machine, extending from the throat plate to the front of the shelf. Make sure part of the tape covers where you want your seam allowance to be. (Ours is at 5/8", standard for commercial patterns.)
2. Using a clear ruler with 1/8 grid squares, place the line for the seam allowance you want under the needle. The right edge of the ruler will be parallel to the guidelines on the throat plate.
3. Draw a line along the right edge of the ruler making sure to keep it parallel to the guidelines on your throat plate (the metal plate under the presser foot)
4. When sewing, keep the edge of your fabric aligned with the line on your tape. Focus your eye on the fabric edge and not on the needle, so you can make any adjustments before sewing with a crooked or too small/too big seam allowance.