We concentrate with the mind's illumining one-pointedness. We meditate with the heart's expanding vastness. We contemplate with the soul's fulfilling oneness.
When we concentrate we are like a bullet entering into something, or like a magnet pulling the object of concentration toward us. At that time we do not allow any thought to enter into our mind, whether it is divine or undivine, earthly or Heavenly, good or bad.
In concentration the entire mind has to be focused on a particular object or subject. If we are concentrating on the petal of a flower, we try to feel that nothing else exists in the entire world but the petal. We look neither forward nor backward, upward nor inward; only we try to pierce the object with our one-pointed concentration. This is not an aggressive way of entering into something. This concentration comes directly from the soul's indomitable will, or will power.
When you want to practise concentration on an object, you should choose something that gives you immediate joy. If you have a Master, your Master's picture will give you immediate joy. If you do not have a Master, select something that is very beautiful, divine and pure, like a flower, for example.