In that case, you'll have to either be daring enough to approach the people you're interested in photographing, talk to them, and ask their permission to make a photograph of them. Or, you learn how to do it by simply taking the picture without asking.
Whatever works for you to convey what you wish to achieve. Try both approaches and see how they work for you. Or maybe there's even a third alternative I haven't thought of.
Either way, I think the choices we make in how we photograph, greatly reflects our personalities. Some people are shy and would not be caught dead asking for permission, while to others that would be a breeze to do and 'What's the big deal?'. Find out what suits you.
To me, it's extremely difficult to muster up the question: "Do you mind if I take your portrait?" Very hard for me. Sometimes I do it, but not often enough.