I arranged mine 0-3, 3, 3-6, 6, 6-9, 9, 12, 18, 24. I find that different companies run smaller or bigger. I just put on whatever fits and when it doesn't fit it gets packed away.
Generally, you can hold up a 3 month to a 0-3 month and tell. Or even the 3 month to the 3-6 month. I do this in the stores all the time. Some brands the 3month size is 0-3 month and on others it is 3-6 month. It sound as clear as mud I know but that is the best explanation I can give. Usually the cheaper the brand the smaller it is. Also look at where it is made. In an Asian country, it will fall in the smaller side. In US or Mexico, it will be on the larger side.
Re: Rookie sizing question
some people say it depends on the brands...but I always put those as 0-3 when organizing.
You'll quickly learn which brands run small/large.
3 mo = 0-3
6 mo= 3-6
9 mo = 6-9
and so on
Emily 8.8.08
Madeline 1.2.11
William 8.5.12