To be effective, the guide can not move. If it has any wobble to it your seam will be off. Just a little bit can make a final difference. I use a long magnetic for much of my sewing. I place it for the correct seam and it is long enough to keep a perfectly straight, even seam. When finished, I pull it off.
For a perfect 5/8" seam, place the tape measure sideways. Most of the 60" measures are a perfect 5/8" in size. Mark the spot! Then stitch out a straight seam and re-measure to make sure.
Seam guides come in every shape and type. There are magnetic, screw on, and even adhesive.
Don't have a seam guide?
- Use painters tape. Use your tape measure and measure over, placing the edge of the tape where you need it.
- Use a rubber band. If your machine is a free arm put the rubber band around the arm.
- Place a sticky note where you need it. Want a higher guide? Stack up the sticky notes!
- Cut apart an old tape measure and tape down (take off the metal end).
However you mark it, keep your eye off the needle!