This one was somewhat relevant to me, since it would be a cool technique for some bellydance shows (since I am NOT willing to get a real tattoo).
The Pintester (nsfw if your work does not like a lot of swearing, btw) tried it and it worked for her. Me? Not so much. And I attempted it multiple times with many different pens, all the results were the same.
Here is the pin/link/instructions I used.

The flower on the right was drawn directly onto my skin using a gel pen. The flower on the left was put there using the parchment paper/water technique.

After one swipe of a wet washcloth (yes, giving both ample time to dry) the drawn on one fared much better, though they both were pretty much gone. A quick scrub with the washcloth and the other one was gone, too.
What might I have done wrong? I don't know why it has worked for some people - I've tried it three times, with three different "gel" pens, and every time I get the same result. I get more permanency drawing directly on my skin with a non-gel pen, to be honest. Could it be the chemistry of my skin? Sure. And maybe it'll work for you. But for me, this was an utter pinfail. And that made me sad - of all the ones I wanted to work, this was high on the list.
The Pintester (nsfw if your work does not like a lot of swearing, btw) tried it and it worked for her. Me? Not so much. And I attempted it multiple times with many different pens, all the results were the same.
Here is the pin/link/instructions I used.

The flower on the right was drawn directly onto my skin using a gel pen. The flower on the left was put there using the parchment paper/water technique.

After one swipe of a wet washcloth (yes, giving both ample time to dry) the drawn on one fared much better, though they both were pretty much gone. A quick scrub with the washcloth and the other one was gone, too.
What might I have done wrong? I don't know why it has worked for some people - I've tried it three times, with three different "gel" pens, and every time I get the same result. I get more permanency drawing directly on my skin with a non-gel pen, to be honest. Could it be the chemistry of my skin? Sure. And maybe it'll work for you. But for me, this was an utter pinfail. And that made me sad - of all the ones I wanted to work, this was high on the list.