Having now had bags of practice (sorry!) I've actually managed to get another two finished today for my own children to put their Christmas present dolls' quilt sets in.
I was whizzing along feeling (over) confident until I realised that in one bag I had omitted to leave two gaps in the seams where the drawstrings should be inserted. Luckily it did eventually occur to me that all I had to do was sew two lines round the bag as per the tutorial, with some extra backstitching as I crossed the side seams to secure them, and then very carefully cut through the seams at the point where the gaps should have been.
Again I can't recommend this tutorial highly enough - these bags are so satisfying to make and I'm already thinking about how my next ones could be embellished.