Practice, I guess. Back when I was doing much singing, there were certain songs that were hard to do, whether on drums or playing rythymn guitar. Syncopated tunes, or "bad writing" with too many syllables :). Sometimes I would "pull" the vocal beat or even the musical beat to meet or make counterpoint so it didn't throw off my timing. Some people are pretty gifted with not needing to do that.

For lead, I think you're going to need to have the lead down so pat you don't need to think about it. I personally can't do it, aside from some riffs between lines or verses. I don't know that I could ever carry out a solo and sing at the same time. Good luck!