At the most elementary level, of course, this famous statement just promises that if you keep practicing the ashtanga vinyasa system that Patthabi Jois devised and taught for many years, you will gradually, posture by posture, make your way through the series and learn the entire system. But interpreting it at this level completely misses out the depth of spiritual wisdom that this simple and straight forward piece of advice contains.

To understand it properly, you  must first look at what it really means to "Do your practice".
Certainly, it isn't going to a class. In a led class, no matter how good the teacher is, the pace and the structure of the practice you are made to do cannot suit every student all of the time, and most likely, will not in fact be anyone's practice, but rather a compromise between several student's practices. Even in a "self practice" class, there are other students who will see you and may even look at you. I know seasoned ashtanga practionners who would rather pretend that they didn't ever notice the look of awe on the face of the pretty beginner behind them, as they effortlessly grabbed their ankles in Kapotasana, or slipped their feet behind their head in yoganidrasana. But if they are honest with themselves, they have to admit, that, if only for a split second, they were no longer practising, they were performing. No matter how much you  try to concentrate on your own practice, in a class situation, you will, at times, be distracted by the other students in the room.  Even in a one to one class, there is still a teacher looking at your practice, giving you instruction, and therefore distracting you. It is only when you are in a room completely on your own that you can truly practice yoga, free from all these distractions. Yoga classes are for learning yoga, not for practising yoga.
And even when you are practising on your own, you are only getting one step closer to doing your practice. You might still, and at times most certainly will,  be doing someone else's practice rather than your own (especially in the beginning). As someone who spent many years doing other people's practices, I have discovered that building your very own practice takes much learning. Nancy Gilgoff, the first Western woman to study with Patthabi Jois, and one of the best ashtanga teachers I have met, says that. with yoga, you are a beginner for seven years. It certainly took me that many years of seriously studying three different yoga systems before I could even attempt to build an asana practice that was truly my own, rather than that of one of my teachers. And I suspect that if I had stuck to one system, it might have taken me even longer to make it my own, so I could actually be doing my very own practice within this system.. Doing your practice require a lot of experience as well as a lot of self awareness, integrity and humility.

We must then look at what is coming into our lives through patient, dedicated yoga practice.
There are many reasons for taking up yoga. Some people take up yoga  to lose weight, others to cure some disease, or at least alleviate its symptoms, others again want to improve their fitness or their sex lives. At the root of what brings us to yoga is, in nearly all cases, a genuine desire to change for the better.  Usually in the beginning we see things in a very narrow way, focusing on a very specific aspect of our life that we are ill at ease with and not realising that to cure the cause of this dis-ease, yoga will be treating the whole person. Only with time do we discover that the flexibility and strength that regular yoga practice builds is not just physical, but also mental and emotional. But once we become mentally and emotionally more resilient, we are more willing and able to welcome change in our lives. Then the awareness of the body and breath that we build through our practice, the tenacity gained by holding various yoga postures for minutes  at a time, will slowly pervade all aspects of our lives. 
The many blessings that yoga practice brings often come in disguise. Often what comes is not what we want, but what we truly need. We have to have faith in the process, trusting that in the end, all that is coming to us  is for the betterment of our immortal soul.