Originally Posted by crispyquilts

All these ladies have great advice. I would like to add a couple of things though...I found that unless I'm in a swap, I really don't care if my blocks are exactly 12.5" square or what ever the size of block I need. I realized that if I fed my fabric under the foot exactly the same way on each piece, all my blocks came out the same size....be that 12.25 or whatever and all came together as a top.

If you really feel you have to have your blocks exact....Cut 3 strips of fabric 2.5" wide by 6.5" long. Measure 1/4" away from your needle, stick down some tape and sew the strips together. Press and measure the square....is it 6.5" square? If not, adjust your guide and try again. Once you have your guide tape where it needs to be, do a couple more to get the hang of it and you should be set to go.

One more thing, using leaders & enders really helps you start and stop in a straight line (as Bellaboo mentioned). Leaders & enders are scrap pieces of fabric you sew first, then sew your piece then sew onto another scrap (leave the ending scrap as it will be your leader for the next piece).
Hope this helps.