Everyone is giving you very good advice for making sure you get an accurate quarter inch straight seam. But a couple of comments: First, if you are getting a lot of "creep", where, usually, the top layer of fabric ends up being "longer" than the bottom layer, the reason is probably because the pressure foot pressure is too tight. Loosen it if at all possible. I firmly (no pun intended...) believe that most machines today are set with the pressure too tight. The pressure foot should be just tight enough against the bed of the machine, with the fabric in between, to allow the fabric to move evenly and smoothly. If it is too tight you are literally pulling the bottom layer to the back with the feeddogs and pushing the top layer toward the front with the pressure foot. This is why you so often see people advocating the use of a walking or evenfeed foot. I learned to sew before such things existed, or at least were available to the home seamster. We learned to adjust the pressure foot pressure for different fabrics. (But to not ever adjust the tension--- ) Today it seems that people are learning to adjust tension, to use different types and sizes of needles, even different weights and types of threads but not to adjust the pressure foot pressure. (and it is much easier to do on today's machines. Most have some sort of dial with numbers instead of a generic "screw" with no gauge related. ) For several years many machines did not have the capability to have the pressure foot pressure adjusted, it was supposed to be "automatic". I think that is when the idea of adjusting it got forgotten. I had one of those machines, and never again will I have a machine where I cannot adjust the pressure.
To test your pressure foot pressure, and your quarter inch seam too, cut
3 strips of fabric, 1 1/2" wide, and all the exact same length. I like to cut a 3 layer "sandwich", so I can also double check my cutting. If you slant your rotary cutter off perpendicular, you will not have exactly the same width/length. Remove the top piece of fabric and sew the other two together along the long edge with a quarter inch seam. The ends should be together when you finish. If you have creep, you should adjust the pressure foot pressure. If you drift at the end of the seam you will see that too. If you are using the full width of the feeddogs evenly, (Pressure foot sits entirely on the feeddogs, and the fabric is also covering all of the feeddogs as you sew) your machine should sew straight with no wavering. However, with many of today's machines, the feeddog area is wider, and using a quarter inch foot with a centered needle position, you don't actually use all of the feeddog on the right side. The fabric doesn't cover that part. Then you have fabric that is feeding uneven "pull". There is more "pull" on the left of the seam line than on the right. Older machines have narrower feeddogs and the pull is more even. But if the mechanism that moves the feeddogs wears unevenly, you may not get that even pull....... sigh.Back to your 3 strips of fabric, 2 now sewn together.... after adjusting the pressure foot pressure, sew the 3rd strip of fabric to the others along one long edge. Press seams away from the center. If you have a 1" wide ruler, (I love my 1"x6" or 1"x12" ruler for this) lay it on the center fabric. It should just fit, and the seams should be nice and snug against it, with no spaces between the ruler and the seam line, and no places where the ruler overlaps the seam line.
Hope this helps some.
Pati, in Phx