Ok first thing to do is to get as many used highlighters you can find. The kind that no longer works, that is all dried up and that you still have lying around. Find as many as you can (Make sure they are all of the same color).
Get a small jar about the size of a mayonayse jar and fill it with warm 1/4 water.
Now take off the cap that is at the bottom (use a screwdriver if you need it) and take out that little cloth like thing that is wrapped around some kind of plastic sheath.
With a pair of tweezers extract the cloth and throw the sheath away. Place the cloth in the small jar.
Repeat the process with as many highlighters you found. Depending on how many highlighters you used (and the size of the jar) you will need to add more or less water.
I found that you should fill it max to 3/4 the way up.
Now close the jar, tightly and shake it.
Let the contents settle for a day or so.
Now re-open the jar and with a pair of tweezers try to pick up a piece of that cloth like stuff which was insed the highlighter and with another pair of tweezers squeeze it so that all the water and ink drain from it. The process is more effective if you use a pair of latex gloves and squeeze the cloth things yourself. MAKE SURE TO SQUUEZE THE LIQUI INTO THE JAR. When the cloth things are mostly white you can then discard them.
What you will be left with is a colored liquid which you can use as 'invisible' ink with a fountain pen (don't use expensive ones, use the ones you can buy in bulk for like 5 for $10). You can draw things on your wall (but start out small where no one can see them in case the ink is too concentrated. (if so add more water).
You should be able to draw on a wall and not see the ink unless a black-light is turned on. If you can clearely see it then it's too concentrated.
make sure to kep the UV lamp (blacklight) on to see what you are drawing.
You can also pour the liquid into an old vodka, rum, or whatever (transparent) bottle tomake it do a cool effect like in bars. The more concentrated the liquid the better.
Hope you enjoy.