What they said....for me it's a matter of listening to the song repeatedly and plonking on my keyboard a bit while typing it into 3mle. I can't read sheet music (Yes i have tried to learn it by myself...it just is like some alien language to me) so I have to do all my compositions as Kurama said 'the hard way'.
But yes, pick songs that you don't mind hearing over and over. Because to sound check you'll be listening to it a lot, even to just get a final edit done to ensure that it's going at the correct tempo with the correct basic sounds and rests. But also to add to advice, sometimes putting your own spin on the song can be quite fun. I quite enjoy making a piece flow the way I want it to rather than sticking strictly to how the piece's composer wrote it down.
PS. Patience? It's not patience! It's addictiooooon~