Sewing a Quarter Inch Seam is important for quilting
No matter what type of quilt blocks you are sewing it is crucial that you have the same size seams for all your blocks so that they can be matched to each other to meet at corners as they should.
The quarter inch seam has become the standard for quiltmaking and almost all books and patterns you will read will assume that you will use this size seam for sewing blocks and borders.
(One quilting author who has a little different approach to seams is Mary Ellen Hopkins whose popular book It's Okay If You Sit on My Quilt Book and others of her patterns explains that you can use any seam allowance you want as long as you always use the same seam on all your blocks. But, for the sake of most patterns, I suggest you stick with a 1/4 inch seam.)
For most sewing machines you can buy a special "quarter inch feet" for sewing quilt block seams, either one made for your machine or a generic type such as the Little Foot. Check with your sewing machine dealer if you do not have a foot for your machine for quilting. Even with a special foot you have to practice with the placement of the edge of the seams to be sewn to obtain an exact quarter inch seam. You will be surprised with the ability of your eye to line up the fabric correctly after you practice, press and measure a few times.
Don't despair if you cannot obtain a special foot, you can also use masking tape or some quilters like to use a thick marker such as "moleskin" (the stick-on padding used to prevent shoes rubbing) as the guide. Experimenting to mark the bed of your sewing machine to allow you to line up a quarter inch from your needle position is the idea. Using a clear quilters ruler, or accurate graph paper, gently lower your needle at an inch mark and then place tape or moleskin marking 1/4 inch away to guide the fabric edges. Then experiment as outlined below and practice will reward you with a quarter inch seam.
How to Test for a Quarter Inch Seam
Rotary cut 3 strips of fabric (pre-washed and ironed) measuring carefully into 3 strips, each 2.5 inches wide across the width of 3 different quilting type fabrics from your stash. You should have 3 strips of fabric each approximately 40 inches long and 2.5" wide, Fabric A, B and C.
Cut these, measuring carefully, into 2.5" squares.
Now sewing with what you hope will be a perfect 1/4 inch seams: sew 1 square of Fabric A to one of Fabric B and the Fabric B to a square of Fabric C to form a row of the 3 squares. Press seams as you plan to use in your quilts, either to one side or open (See this article about using open pressed seams for more on this.)
Measure across the 3 squares you have sewn together. If your seams are 1/4 the 3 pieces sewn together should measure exactly 6.5 inches. If they measure less, you have sewn a slightly over 1/4 inch seam, if they measure more, you have sewn seams a little under 1/4".
Adjust your sewing and make another set of 3 into a strip of 3 and measure again. Notice closely where the edges of your fabric are in relation to your quarter inch foot or tape or however you marked your machine.
If your first block came out perfectly - Great! Do 2 more sets and make sure you can reproduce this.
When you have finished 3 strips that all measure correctly, experiment with putting them together to make a Nine Patch Block. It should measure 6.5" x 6.5" when you have sewn the 3 rows together and the corners where the rows meet should match nicely.
If you are beginning a new quilt with a fabric that is a little heavier or homespun type, you may want to do this seam testing again as a thicker fabric can affect the quarter inch seam since a little more fabric is used in the turning of the fabric in the seam.
Keep practicing - it gets easier and easier and your eye gets better and better at lining up the seam.