also, remember: it's the size of your block that is the most important, not the size of your seam.
Your aiming for a 1/4" seam, but a few threads this way or that, is not a tragedy.
To get your seams etc. to line up, you HAVE to have an accurate block size.
I like to draw a sewing line on the back of my blocks sometimes, with a mechanical pencil, as lightly as you can, but still able to see it to sew on.
Using the corner of your rotary ruler or a T square will help you get your block accurate.
The freezer paper suggestion was very good.
I don't want to speak for BellaBoo, she may come back and explain what she meant.
I would use the corner of my ruler to get, say a 4" line going to the right, and on the left of ruler, go 4" down, (4" is just an example). Then I would fold the paper in half diagonally and draw the other half of the block.
Then I would iron the paper on to the back side of the piece of fabric and add my 1/4" seam allowances.
Then either sew right next to the edge of the paper, or draw around the paper for a sewing line.
If you draw the line, remove the freezer paper and I would sew one thread inside the line.
Another thing I do, that I will be doing ten years from now: sew a block and measure, sew and measure...
Better to rip out one block, than a whole row, or even more :shock: