Blue eyeliner pencil would be perfect because its thin enough. Take the eyeliner and draw veins on your legs, if you look on your inner wrist you should be able to see some, mimick that. Remember though that they are under the skin so the lines cant look too perfect...use a smudger to smudge them and apply a thin amount of powder makeup on top to make them transparent.
So i'm cosplaying as a nurse from silent hill , I noticed that they have veins running down their legs , how can I draw them onto there ?
-looks at my wrists- ah there they are , thank you :D this is going to be very helpful ^__^