For the second lot of undies, to make things easier, I took one of the needles off my overlocker and used only 3 threads instead of 4 - what a huge difference this made to the neatness of the seams.
The only problem that I had with the first pair, is that the ribbing would slip and so when I turned out the garment I found that I had missed picking up the ribbing in some parts - to fix this problem, I had a larger seam allowance and made sure that when the ribbing was folded in half, the bottom stuck out a little bit further than the top.
This time I used some of my old t-shirts. Actually these t-shirts have been sitting in my wardrobe for years and have not been worn, so they are still in new condition, and now they are not going to waste.
Too bad they are not a little bigger - or I would wear them myself!