If you have a map idea already, try to find a good picture of the internet of it.
Then use the Map Making section of the wiki to learn how to do the inkscape things and such. You'll pretty much have to use all of the pages to understand, but the most useful are 'Inkscape tips', 'Guidlines for Making a Great Map' and 'Creating the SVG'.
The Wiki: https://www.warlight.net/wiki/Map_Making
Edited 9/20/2015 10:08:58