Tips for Stitch-and-Flip Blocks
Many common quilt patterns use the stitch-and-flip technique. To use this technique, you draw a diagonal line across the wrong side of a square or rectangle, place it in the corner of another piece of fabric with right sides together, stitch along the line, then trim and press the marked piece open, (Get it? Stitch and flip?) Here's an example of a stitch-in-flip pattern, below.
Here are a few stitch-and-flip tips to get accurate pieces:
Draw your line with a thin pencil or fabric pen. The thinner the better. If your line is really thick, you might sew at a slightly different angle because you can't tell which part of the thick line is the angle you want. Use a good light as you're sewing, so you can stay as close to the marked line as possible. How accurately you follow that marked line makes a difference in what the finished piece looks like.
Sew a scant 1/4". There's a natural bump where the "flip" or fold is in the attached fabric piece, which may cause your attached piece to turn out smaller than intended or not line up correctly with other units. If you're finding this to be a problem, try sewing a scant ¼" seam, sewing approximately two threads away from the marked line into the seam allowance. That little bit of extra room will get used up in the bump when you press. It might take a little practice, but it should help you piece units that are the correct size when stitching and flipping.
Snowball blocks are common units that use the stitch-and-flip method. See free patterns to practice with here.