The instructions are crystal clear (but check here for the one amendment). I ticked them off in pencil as I completed each one. But I do have some tips.
First tip: do not buy new hardware for this bag - pop into your nearest charity shop and I will 'bet-my-bottom-dollar' that you can find an old bag with the necessary tri-bar slider and rectangle loop. This is exactly what I did; the first charity shop had a bag with, what looked like, the right size equipment. I handed over my 99p and rushed home. Yes, it was the right size and there were two tri-bars on the bag along with the rectangle loop. I even managed to salvage the brown mesh handle for another project later on maybe!
Bluepatch Quilter had made the bag before jetting off on her hols and said she wished she had made it a bit stiffer. So I took her advice and used a heavier fusible interfacing. Three needles later and the strap was done! But she was right, the strap, the flap and the bag are better for it.
Fourth tip: leave AT LEAST six inches to turn the bag through - think Zakka pencil case and you will know what I mean.
As I walked in to show my daughter she was shocked. "How much of that did you make and how much did you buy?" she asked (she had seen me come home from the charity shop with the old bag). "I made all of it," answered I, "even the firm bottom!"