I did this for halloween several years ago, and it has survived well enough to be used through multiple halloweens. The effect is just fabulous, it really does look like you have a head floating in a jar, from all angles. I BELIEVE i got the idea from this site, but I’m not positive, and he has taken down his tutorial on making it, due to theft (why do idiots ruin it for the rest of us!?).
So, here’s what I can remember about how I created mine.
You need a suitable jar. Mine has a pretty wide mouth (it’s a SunTea jar from the thrift store), but it looks even more bizarre if there is a small mouth, so it appears that there is no way you could have got the head in there.
Most importantly, you need a picture. This is the key. It needs to be a flattened image of a head. Something like this or this or this or the woman here. (This is from a quick googling. I think I found mine looking at stuff for 3D rendering of face textures, and 3D scanners. You’ll probably want something creepier, so use that google fu.) (edit: good one here.
Then print the picture on the heaviest, slickest paper you have, at a size that will allow it to fill most of the jar. It will work on regular paper i would think, but it may not hold up. Roll the paper up, with the face facing outward, and insert into jar. The paper should be big enough to fill about 3/4 of the circumference of the jar, so as to give you a full 3D view (Note, this depends on the size of your jar).
Fill the jar with colored water so it’s murky. On mine, there is a little space between the picture and the jar sides–I’m not sure it matters–you just want the head to look the right proportions. You may need to stick something inside of it to keep it in place. Just mess with it until the effect looks pleasing.
I then used some hair from a doll wig on top of it to simulate hair. This adds to the 3D effect, as it’s actually 3D.
The night of the party, i threw a few glow sticks in there to add to the ambiance.
And that’s about it! Enjoy!