I am going to do a presentation for my guild at some point in the future, and part of it will be how to sew an accurate 1/4" seam. As practice I thought I would do a short tutorial here for anyone who has difficulty.
It took me a while to understand the importance of the "scant" quarter inch seam. When sewing simple blocks like a 9-patch, it isn't so crucial, as long as your seams are consistent (and you're not participating in a swap!) If you are sewing a more complex block with half-square triangles and other shapes, if you don't sew an accurate seam, your pieces will not match and you will end up frustrated and decide that you just have to stick with the easy blocks. If your block has lots of pieces and more than just squares and triangles the effect of your personal seam allowance will be magnified.
Here is a short tutorial of how you can check your seam allowance, and adjust it so that it is an accurate quarter-inch.
1. cut three strips of fabric 1 1/2" wide and about 4-6 inches long. It will be easiest if there is some contrast between the middle strip and the edge strips.
2. set your machine to sew your quarter inch seam.
If your middle strip doesn't measure 1 inch, you have to make some adjustments to your seam set-up.
If it is larger than 1", your seams are smaller than 1/4". Measure how much larger than 1" you have, divide by 2 and move your seam guide away from the needle that amount. For example, if your middle strip measure 1 1/4", you need to move your seam guide to the right (away from the needle) 1/8" (1/4 divided by 2 equals 1/8)
If you measure your middle strip and it is less than 1", you need to move your seam guide closer to the needle half the amount. If it measures 3/4", move the seam guide 1/8" closer to the needle.
If your machine allows you to move the needle, you can move it instead of the seam guide. Make as many test strips as you need to get an accurate 1/4" seam. Make sure you note how your machine is set up so you can duplicate the settings.
Hope this is clear (and useful!)