I like that the word practice is in this Maxim. The word ‘practice’ is a reminder that there is no perfection, and Justice is a Platonic concept – something that seems like an Absolute but in reality has many nuances. So we aim for what is just in our actions and intentions.
Unfortunately, the concept of justice seems to be one that is confused and muddied. Justice is equated with what the courts decide, and sadly, courts are often not just. Racism, sexism, classism and other -isms obscure what is just.
You might have seen the cartoon below making its rounds through the interwebs. I think it sums up the concept of justice nicely.

So, how do we practice what is just? The world is so vastly un-just that I can easily get overwhelmed with all the ways my existence perpetuates injustice, whether economic, environmental, or some other form. I also know that since I am not actively working to dismantle injustice in certain arenas, that I am maybe perpetuating the problem that way. For example, I am a white, middle class, highly educated person. I am aware of the gross racial injustice in our criminal justice system, but I am not actively doing anything about it. In my daily life I have no interaction with that system. But the little I can do is educate myself on the issue. I can raise my children to be aware of the issues and to understand the ideas of privilege. I can vote.
In other areas, I contribute to community causes, I try to make the best decisions I can for my family regarding food, clothing, transportation, toys, media etc. But hell: I use a macbook and have an iphone – I am readily aware that there is little justice in the technology I use.
There is no black and white rule book for justice. It’s the boring work of small choices that make it easier to stand for the larger issues that helps us practice what is just.