Getting a 1/4 seam allowance on older machines

Draw two lines on a piece of paper 1/4 inch apart. Be exact.
Trim along the outside line...carefully.

Be exact.

Place the paper on your sewing machine with the needle position right on top of the line.

Be exact.

Sew along the line, with no thread, to make sure the paper is lined up properly.

Place masking tape along the edge of the paper.

Be exact.

Remove the paper and make several layers of masking tape for a guide for the fabric.

Be exact.

I guess you have the hint by now that Exactness is important in every step of the quilting process.

Getting a 1/4 seam allowance on newer machines

Adjust your needle position to where you think the needle is 1/4 inch away from the edge of your presser foot. Now measure it to make sure. Use the edge of the presser foot as your guide.
If you need a reminder like me put one on your machine somewhere.
If you forget you'll be doing what I did...picking out your stitches.