Position the seam allowance guide:
- Place a rotary ruler under your needle, positioning its first 1/4" mark just below the needle's tip. Check position by lowering the needle slowly by hand until it just touches the mark on the ruler.
- Make sure the ruler is positioned in a straight line on the machine -- compare placement with nearby grooves in the throat plate.
- Place a 1" piece of masking tape on the throat plate, its left edge flush with and aligned against the right edge of the ruler. Remove the ruler.
- Sew another test unit, guiding the right edge of your fabric along the left edge of the tape as you sew. Press and measure as before.
- If seams are accurate, stack additional pieces of masking tape on top of the first to build up a ridge for fabrics to butt against, or obtain height quickly by rotary cutting a piece of adhesive-backed moleskin and applying it to the tape.
- If your seams are still off, move the tape and try again. Continue tweaking and testing until seams are accurate.
The feed dogs in some sewing machines are spaced widely enough to prevent use of a guide, because it would cover them, but those machines generally come with a quarter-inch presser foot.