Often times, I start with a 600 dpi scan of a logo I have acquired from either Ebay, a hobby show, or another source. After some tweaking, and tracing in Adobe Illustrator, the final product, shown below, is completed.
Here is the completed logo. This particular logo is for the Chicago & North Western Transportation Company
To the left is the original scan of a 1941 C&NW Timetable that was cropped and imported into Illustrator.