Sewing is not my idea of fun,
so I had a mooch around to see what was happening with seaming.
Came across this site:
(Skip the first three short paras and go straight for the first photo.)
It has very detailed pix of Mattress Stitch and tells you the best way to go about it - for left-handers too.
It even tells you how to fix wrongly counted rows of ribbing and also helps with Garter stitch.