As I understand them, they are seven syllable phrases or sentences taken from scripture. I don't know enough about them to tell you exactly how to use a seven syllable Christian meditation. I believe they are repeated in a rythmic pattern. Accents on certain syllables, perhaps. As I said on the Meditation page, there are many ways to meditate, choose one that suits you.
Christian meditations are one option.
What I teach on the previous page is a relaxation style. I suspect that the seven syllable Christian mantras are used in a concentration style meditation. I doubt the method is the same. That is, I doubt you can just use a Christian mantra with the method I described previously. Monks and nuns and others deeply committed to religious introspection also meditated, although I'm not sure of the methods used.