I would first off say to experiment and see what works for you. For me, I find that sympathy is how it begins, and the deep imprint of their suffering feels like it awakens me from my indifference - the same indifference which is at the root of my own suffering, without my knowing. There isn’t really much of a prayer other than to gain the power eventually to actually emanate everything that they need - like mentally preparing you to become the “Three Bodies” of the Buddha.