Dotting tools are great for creating fun designs. They are availble on ebay or from amazon for a couple pounds...
...but it is not necessary to buy them. Dotting tools are easy to make from household items.
The simplest one is a sewing pin. I like to use an old pencil to help secure the pin and make it easier to use. Simply stick the pin into the rubber and Viola! a perfect dotting tool.
I'm sure I don't need to say this but don't attempt to drip your dotter straight into your nail varnish bottle. Put a few drops on a piece of paper and dip into that.
For different size dots use things like bobby pins, old biro or pencils.