I had lots of fun playing with nail varnish and water. I found that you can create a couple of different styles, very little polish will create a much lighter marbling effect that is easier to handle.
Obviously you can't wrap up paper in sticky tape - as per the tutorial for marbling on glass beads. So I just dropped it on the section I wanted to use and then carefully picked it up again. I had already used my paper punch to create the heart shaped pieces of paper, but I'm sure you could marble first and then cut the paper to the shape you wished, well once it had dried anyway.
It is tricky not to get it messy with the polish around the outside, so I would recommend moving that out of the way to the side of the cup while you take out the paper. I also think a pair of tweezers would allow you to be more accurate, but I was too lazy to go find mine at the time.
The kids had a go too and they had a lot of fun. We did discover that certain kinds of polish will eat through the plastic cups if left long enough. This only really happened when the amount of polish dropped into the water was so big it didn't stay on the surface, but dropped to the bottom. We ended up with water everywhere! I was pleased it was only water and by that stage I'd put newspaper under the plastic cups so that any polish spillage was caught on that.
I think I'll be using this technique for all sorts - cards and jewellery making, I think it would be great for scrap booking too. I wonder if you could do a whole sheet if you had a bigger tub to do it in, or whether it would dry too quick. That might be an experiment for another day!