Van,
There's a number of ways you can transfer patterns onto brass. One way is to copy an image onto a piece of transparency stock (like for an over-head), coat the metal with some acatone, let dry, then rub the image onto the metal. If you're not familiar with it, google "The Engravers Forum", and search for transfer methods, they have great stuff.
Most of the time I use a mechanical pen. I'm not sure of the size off the top of my head, but think it's around .02. The smaller, the better. Just like carving, I think it's really important to have fine, definitive lines to engrave from. Hope that helps....
Ed