I made my first flat-felled seam by hand yesterday - just a sample. I followed two sets of instructions I found on the web. Here are the links:
There is also a video from ExpertVillage on youtube.
I don't have David Page Coffin's book Shirtmaking yet; I have to wait for my new credit card to arrive first. It is high on my list, as he seems to be overly into the fine details of sewing and I recognise a kindred spirit there! He recommends (nay, demands!) using a felling foot, as described by kbenco in this Patternreview post. If you have a felling foot, Nancy has a good tutorial on PatternReview.
The first row of topstitching in my sample is a bit wonky, and I plan to remedy that by trying to sew from the other side instead of using my edgestitching foot to follow the folded-over seam allowance on the wrong side. Maybe use glue stick to secure the seam allowance? Of course, it would also help not to use a contrasting colour.