Loudly chant the mantra. Then, slowly moving your lips, recite the mantra. If the mind is wandering during mental recitation, then again start loudly chanting the mantra. Actually, japa itself is a complete sadhana. That, by itself, is sufficient to purify your mind and allow it to concentrate on the divinity of your mantra.
Every mantra has an originator, which is a rishi .
A divine sage visualised the mantra in his meditation, and so his thought also is present there and is charged upon the mantra. The book is only an instrument, a guide, to keep you in harmony with the thoughts of the great author of a scripture or book that you are reading. So, while chanting the mantra, you are in harmony, en rapport, with the great thought of the sage who visualised the mantra. The mantra itself is a power. The words of the mantra are not mere haphazard letters.