I just tie the regular ponytail scarves in a bow (like you're tying a shoe) and then I play with it to make it all even and pretty. For the square scarves, you take a corner and then spin it in the air so it becomes rolled up and then you just tie it around the bag, usually not in a bow.
^the big square ones you can take one corner and fold it so it is pointed towards the middle of the scarf, go to the opposite corner and repeat. then fold it again, towards the center, and repeat with the other side, adjusting the width of the scarf with how many folds you do..
if you think of it like origami, i think it makes more sense (what i said above)
There's a thread about this that helped me out (someone not as "search challenged" as me should be able to find it) But like the others said:
Lay your bag flat on a table with the handles facing you then tie bunny ear tie the bow. Then when you turn the bag right-side-up it will look nearly perfect everytime.
When I was still not so good at it, I would fold the ponytail scarf in half lengthwise and tie it to the handle/ring in a knot. It has only one loop, but if you make the loop side shorter, it looks really cute when you fold the loop forward towards the end of the scarf. I hope that makes sense...