Once we have achieved a level of pratyahara and can withdraw from the distraction of external stimulation, we now can more easily work on concentration.
The monkey mind is always in motion and very difficult to tame. Concentration is the next step towards our path to bliss through the process of corralling all those monkeys.
One of the exercises we can do in our sitting practice to begin the monkey taming practice is to focus on an object. That object can be the flame of a candle, rock, or mandala.
There are several approaches to the concentration practice. One way is to allow the mind to wander within boundaries. So for example, you focus on staring at the flame. Then as your mind begins to wander, you discipline it to only think about flame. Every moment it drifts away from the flame you bring it back with an observation or analysis of the flame. The goal is to begin to see the flame but not process any information. Much like the exercise of pratyahara, your sensation of the candle becomes disconnected. You see it but you no longer process it.
How do you know you have achieved dharana? In the beginning, it may be very frustrating. It may seem that you only get a few seconds of concentration at first and the mind is off to the races. But that is why it is called a practice. Just keep exercising until longer segments of concentration become easier. If you can focus and concentrate on an object for twenty minutes, you are ready to take meditation to the next step.