Heya
Im forever doing the water marbling, I was 'taught' the idea by another nail tech called Naho - she does FAB watermarbles and she's a LOVELY lady!!
I drop a colour in the water and immediately drop the 2nd one ON TOP of the prvious colour. I wait for them to spread out a bit, take a cocktail stick and swirl them together. When it looks ok, I dip my nail in, wait til the rest of the varnish on the water surface to disintergrate or I brush it aside with my stick (so wen I pull my nail out it doesnt get a 2nd coating of varnish!) then take out my finger.
Thats how I do watermarbling, it takes practise and its messy but with time, you'll be a pro ;-)
To see my watermarbles, click here http://kookynails.tripod.com/kooky-nails/id15.html
To see Naho's watermarbles, click here http://www.geocities.com/nahoahlberg/watermarble.html
http://www.geocities.com/nahoahlberg/wm2.html