This plastic seam guide by Lori Holt of Bee in my Bonnet is an acrylic template for measuring accurate 1/4" seams and for constructing easy corner triangles.
Be sure to click on the video tab for how to use this seam guide. Set up for the guide starts at the 5:15 mark.
Set up is easy. Center it, align needle with dark center line. Use the grid to ensure the guide is straight and put the stickies under your template. It will save you time by not having to mark your fabric when sewing half square triangles, flying geese, or snowballs. The center circle of the guide is marked with 1/2" grid lines for projects with a wider seam allowance. Four stickies are included to temporarily adhere to any sewing machine or serger. Washi tape is recommended for any subsequent use. [Note: Always use caution with any adhesive on your vintage Singer Featherweight. If you are concerned at all about the machine's clear coat, then you may need to consider other seam guide options.]
Lori had a Granny's Garden Sew Along featuring her Granny Chic Line. It will be hosted over on Lori Holt's blog, A Bee in my Bonnet.