I never sketch. I prefer pen, because I pre-draw the image entirely in my head prior to placing it to paper. I never erase. I start drawing away and if I make a mistake that I can't hide, I start again, but usually don't have to.
Just because I draw it in my head, doesn't necessarily mean the final outcome is exactly how I saw it, rather it just gives me direction and the artwork finishes itself. If it doesn't work, I try something else.
I am no linguist, however, like cartography, its something that interests me, so I know little pieces and parts of words from various languages. I research anytime I need more data. However, I choose from what I know, then look at an ethnic regions words that describe town or river and add them to the parts I know.
Sometimes, I completely make it up out of my head, say it outloud, to see if it sounds interesting. If it works I use.
More often than not, I plan my map site names like I plan everything else - in my head.