Like anywhere in the art world, you need to start off work with the basics. Not drawing basics mind you, that's further on, but the basics of creativity for others, Which is simply, ask. Whenever designing something for someone else, common sense dictates you kinda need to know what they want and have their input before starting.

Now that you have the basic design drawn out, it's time to pick a spot and start drawing. Whether it be a freehand or stencil (I'm more partial to making a stencil) you need to make sure the size is the right portion for the art. With this, reducing the size makes it fit just about anywhere. After drawing the basic design, you need to find the type of marker or pen to fit your skin type. I have found that fine point felt tipped gives consistent lines, and some are non toxic for use on skin.

From ball point to felt tipped, all pens are not created equal. Certain pens are necessary for the type of media you are using, whether it be parchment or A4, to fake and real human skin. Each carries the ink a different way, each texture has qualities that are unique to that specific texture and nowhere else. You have to try different styles of pen to find the one that suits you and your craft the best, plus looks appealing to the design.