When I have done it I have just had to free hand, takes practice though (I know I need more!). I will either put the icing in a plastic baggie and cut a tiny hole, or use a piping bag with a tip on it.
I just use royal icing (although it doesnt taste right with the cookies that are soft in the middle). I make the regular consistency for the outline part, then thin some of it out so I can kinda of pour it into the outline with a spoon and spread it around, then let is dry and harden completely. Then you can use the regular consistency icing again to add details (again, AFTER the middle dries).
I am by no means an expert, this is just my experience with it. Good luck! I was actually thinking about the same idea for Easter cookies. 
I use a slight variation of Toba's Glace icing for all my cookies. I usually add more powdered sugar, until I get the consistency of slightly thick Elmer's school glue. For the Easter egg cookies I recently posted, I outlined the cookie using a #4 tip and then immediately piped more icing in the area in a zig zag pattern. I used a tapered off-set spatula to smooth out the icing over the entire area. I thickened my icing even more, until it was the consistency of peanut butter. I used a #1s or #1 tip to pipe all the details (crosshatching, dots, etc.). Normally, it would kill my wrist to do all that piping, but the pastel colors seemed to flow differently than when using black. These eggs were a LOT of fun to do!
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