The process is a bit labor intensive as I work with my clients on defining a style and then clarifying what landmarks will be included. Once this has happened, I sketch out the base area and then separately sketch out the landmarks. I then impose the landmarks onto the base in photoshop. This way I am able to edit individual items as needed rather than having to re-design the whole map every time a change is requested.
One of my very favorite things to do is to design and paint custom maps for my clients. I just love the character that a custom map can add to a wedding invitation suite or to a welcome bag/basket.