New to metta meditation

I have some questions about metta meditation. 1- When I send love to myself, my cat, the world etc. Do I have to feel something? Do I have to sit and repeat the mantra May I Be Happy until I feel love for myself and not move on to my cat until I've reached that point? (If yes, what if that takes forever? If I don't feel a thing, why might that be? Do I move on anyway? How much time should I spend on each thing?) 2- When I imagine myself, my cat, my neighbour and so forth, how much energy should I put into visualizing these things? Do I need to see it all clearly or is it enough to keep them in mind?