Sewing seams is easy so long as you remember a few handy tips:
a) block your work
b) block your work
c) block your work (got it)?
I used a moderately hot iron (set to "wool") and steam, iron very gently (with almost no pressure) and let it rest.
For mattress stitch I always begin on the wrong side by threading my yearn through the edge leaving a tail of about an inch poking out, then bring the yarn up to the right side where all the rest of the work commences.
It's important to realise what mattress stitch will do: it will pull two edges together (no overlap) and if you decide to do it a column of knitting in, will form a ridge on the wrong side (which is cool).
I always sit in very strong light (preferably sunlight) so I can see exactly where my needles going.
My two cents' worth ...