There are many versions of the voodoo magic trick. What you are trying to do here is simply to make the person believe you know what they are thinking of or what they had written down. One example is telling the person what they had written down on a piece of paper.
For this, you will need to round up somebody and ask them if they want to perform a magic trick. If they are more than happy, tell them to write something in a piece of paper. When they are finished, you instruct them to crumple the piece of paper and put it on the ashtray.
You then light it up using a match or a lighter and when it is completely burned, you get the burned piece of paper and rub this on your arms. When you are done, you tell the person what was written on the paper.
The key to making this trick work is being able to look at the paper before this is completely burned. You should do this without the other person noticing so they wont know how you did it.
A similar trick will be letting a volunteer think of a number between one and one hundred and saying this out loud. You then show the piece of paper with the same number written that will really make them believe you...