In the beginning you can chant outwardly, then shift your practice towards chanting mentally. Then meditate on the silent background from where the sound emerges or is contained, that is, the silent inner space within you which is consciousness or awareness.
How mantra meditation works?
By repeating a mantra, you are diverted from the thoughts inside your head and your focus now shifts toward repetition. Each thought has an effect on your emotions and mood. It is the nature of the thought to create multitude chain of thoughts. Higher the number of thoughts in your head, greater is the stress. And lesser the density of thoughts lesser is the stress.
When you repeat outwardly, there is every possibility that you might eventually be doing it mechanically i.e, your mouth repeats the mantra but you go on thinking subconsciously inside your head. That is the reason you should shift your practice towards mental repetition. If you are comfortable repeating the mantra inwardly from the beginning, it is very good.
Now after the mental repetition you minds gain the ability to not get trapped in the thoughts. But this mental repetition is not the final goal. You have to be at peace without any effort (effort of repeating mantra) but at the start effort is absolutely necessary.
Then, shif the practice towards the silent inner space (or consciousness) that contains the sound or the source from where it emerges. Constantly focus on the the inner space throughout your day. Be with or focus on the silence of the inner space (i.e, move towards thoughtlessness). With gradual practice of this final step your thought density reduces over time and can achieve a thoughtless state temporarily. Which regular practice you mind gains strength to remain in the silence of inner space and it becomes an habit. Then eventually you can continue to remain in the blissfulness of the inner space without any effort. This absolute peace is the final goal. This is the state of the enlightened and not the repetition of the mantra.
Don't over do the practice, take some time in the morning afternoon and evening and do the mantra japa and go on with the normal activities of yours.