This is as simple as it can get with a stylus for capacitive touchscreens.
1) Get a wood pencil,
2) Gently chew the rear end of the pencil (if you haven't done that yet, anyway) until the lead is exposed and rounded (see close-up picture).
3) Use it as a very precise stylus (the screenshot is from Autodesk Sketchbook for Android on a Samsung Galaxy S5 Active). My screen doesn't get marked, and if so happens it can easily be wiped off.
I ignore the science behind, but I suspect that the fact that my pencil doesn't have a layer paint make this concept work. I have also tried to use the sharp end of the pencil, and although it works the "contact" is inconsistent.
Note: You can also use this stylus on regular paper!!!
Step 1: Update
I am now wildly hypothesizing that the reaction of the screen to the pencil depends on the humidity of the wood. Why? because when it didn't work, I simply licked the tip of the pencil making sure the wood gets wet, and voilá!