Stretching the balloon out first won't make a jot of difference. Inflating it with a pump first will make it a lot easier to re-inflate by mouth, especially while you're 'learning'. It's one of those things that you'll suddenly find 'your way' for. Best advice I can give is to 'spit' the first bit of air into it, once you get that first bubble it's much easier. Don't stretch it out while you're blowing, it's hard enough to inflate a long thin tube, why make it longer and thinner?
It's definitely a knack thing - if a skinny four foot something girly can do it with a partially collapsed lung, I'm sure you'll manage!