First of all, gather your materials.

Transfer the figure to the tracing paper using your sharpie.

Using the cotton, put some alcohol on the skin.

It don't have to be necessary alcohol, it could be nail polish remover or another thing that you have on your house (need to had some alcohol in it).

Tape the tracing paper upside down to the part of your body. Example: The arm.

Gotta be the ink upside down x)

Now rub the tracing paper using the cotton soaked in alcohol.

Take off the tracing paper. You got a stencil at your skin!

If it's messy and some lines are not deffined.. you can clean a little using a Q tip soaked in alcohol.

The only thing you gotta do now is remark the lines using the sharpie.

And that's all, pretty easy, right?