The first step in accurate, easy piecing, is to cut the fabric accurately. Not a smidgen bigger or smaller - cut the size required. Here, my project calls for 2 1/2" squares.
Use a Quarter Inch Foot
Most sewing machines come with a 1/4" foot, or allow the needle to be moved side to side for adjustments. My machine has both, and I move my needle a few spaces to the right to achieve that quarter of an inch between the needle and the fabric guide.
Measure the sewn unit
The final test of an accurate 1/4" seam is to measure the finished unit. Press the seam to one side, then measure the unit. Remember that seams always take up 1/2". In the photo, two 2 1/2" squares sewn together results in a unit that is 4 1/2" wide.
If that unit is too short or too long, first double check the cut size of the pieces. Then go to the machine and adjust the needle. Can't move that needle any more? With the help of a ruler, place a piece of tape on the machine bed to mark where the fabric edge should be. Then sew and measure the unit again. Keep adjusting the needle (or tape), and checking the final unit until an accurate 1/4" seam is achieved.
With all of those accurate seams, enjoy having block pieces that fit together easier, taking a lot of frustration out of quilting.