I have this book by Karen Morris called Sewing Lingerie That Fits. I never attempted briefs/knickers before, but used to make two piece bathing suits way back when I was skinny and it didn't take much to measure.... I was very apprehensive at first, because my "mature" figure isn't what it used to be, but the book explains things very well. It shows you how to make a pattern from a few measurements and it takes no time at all to sew up a pair. Also, there is a tip on how to keep the briefs/knickers from riding up in the back.
I have made six pair so far, a half slip, a full slip, and a camisole; all from the book. I love being able to make those high-priced briefs one would find at Victoria's Secret (not in the window, of course) for a fraction of the cost. I found a really nice nylon knit in beige, and bought a grab bag of lingerie elastic that, interestingly enough, had enough elastic in the same color as the briefs.
The publisher of the book is Taunton Press, and I was fortunate enough to find it online for half price. Best investment I've ever made; can't talk enough about it.