In Serious Eats, the author provides some insight on how to do this:
The process was easy: onions browned and then removed, meat and water
added and boiled, then the rest of the aromatics go in, skimmed
occasionally and simmered for six hours. By the time the broth was
finished it was incredibly phở-like—the cinnamon and anise came
through beautifully and the stock was insanely meaty.
The next day I took off the layer of fat and set about assembling the
rest of the phở ingredients. I got my hands on lovely basil, bean
sprouts, and some shabu-shabu beef.
Source: Serious Eats
Additionally, Viet World Kitchen provides some great basic techniques for building your pho broth:
Roasting the bones.
I've tried this and have not found that it's done
much to the broth aside from making the broth dark, something that
I've not found to be attractive. If you start with good bones, there's
no need to roast, as the French would do for a veal stock, or
Source: Viet World Kitchen, 2007
Viet World Kitchen has so many great articles about pho as well:
If you're looking for more resources loving pho has a great article about pho:
There is no single perfect technique for creating good pho broth. The
Vietnamese always say that the best pho you will ever taste is the one
cooked by your own mother (plus maybe one or two favorite
restaurants), and she will have her own ideas of how good pho broth is
made. These ideas, in turn, are ones that have been handed down to her
by her own mother. Thus, no two bowls of pho made in two separate
kitchens will ever taste the same.
Source: LovingPho 2009