Answer: The same way you brand anything else — by assigning and supporting the qualities you want the public, prospects, clients, partners and employees to associate with whole, above and beyond the qualities that are associated with individual people and specific events and elements of the firm.
One decent definition of "brand" is "corporate personality." So, the question becomes, what is the personality of your firm? What is it that sets you apart from other firms when you all get together to practice law? In many cases, for many firms, the answer is, sadly… nothin’.
Many firms have no rhyme or reason for their joining together beyond sharing office space and copiers. And that, frankly, makes a lousy personality. The truth of that brand could be summed up as follows:
Smith and Jones: We’re convenient for us.
Doesn’t that ring true for many firms you know of?
You can’t "build" a brand out of nothing. You can, though, decide to market a brand, and change a culture to fit that marketing image, but it must be based on real qualities.