I use a ruled 3x5 index card to set my machine to 1/4". The lines are 1/4" apart. I normally sew with a scant 1/4" because of the fold of the fabric. To measure this, for normal fabric, I position the needle so it touches the line barely to the right. Then I put a piece of the neon tape in front of the foot to mark the position. Also, at the beginning of every project I take 4 squares of a known measurement (1.5", 2", .... depends on what I have laying around), make 2 2-patches and measure them to make sure are the right size (if not, I rip adjust, and start again), then I sew the 2-patches together and make a 4 patch, measuring again.
Common errors, the first few stitches and the last few stitches are not accurate because folks anticipate sewing onto the block or sewing off -- that is why using leaders and enders (or sew ons and sew offs as we called them when I was taught) help. They are not only for keeping points out of the feed dogs but are also good to help folks keep a straight seam. I heard Mary Ellen Hopkins say that the most important stitches when you piece are the first 3 stitches and the last 3 because these are the ones that allow you to match up the seams.
Also, I would suggest new quilters learn the joy of starching since the stiff fabric is easier to sew accurately.