(This is my first written tutorial, so please bare with me. :-} )

First off, this tutorial assumes you can create a basic E6/1 top, and that you have a working knowledge of chainmaile in general.

Here's what the rounded out cups of the underwired top look like:

Before I added the 'boning', the top laid very flat against the chest, and gave no support.
The boning was added to the front of the piece, as shown below:

**NOTE: Adding the smaller rings in this fashion will 'cinch' together the larger rings, which will create a tighter weave in the areas that you require more support; thus creating a 'boning' effect. This is the reason the smaller rings are placed on top of the piece. If you use this weave on the 'inside' of your top, it will cause that area to 'buckle out' instead of 'cinching in'. For example, the smaller rings placed on the outside of the top, along the bottom of the cups, cinches together the area under the breasts and creates an underwire effect.

In this case, I am using 18 gauge 1/4" ID, boned into a 16 gauge 3/8" ID E 6/ 1 pattern.
I've not yet tried this with any other gauges or ID's, but I would suggest if you wish to use different sizes then the ones I used; for the smaller ring, choose an ID that fits inside of your larger rings, and about 1-2 gauges smaller. This is just a guess, so feel free to experiment.

The 'boning' goes through three rings at the connection point of 2 rings from the bottom row and one ring from the top row, like so:

The boning rings do not go through to the bottom of the piece.
If you've woven it through correctly, you will not see the boning rings if your turn the piece over to it's backside. Weave in the smaller rings along the connections in your piece that require a 'boning' or underwire.

This is where I wove in the underwire into the above top:

And this is a diagram of all the place that have the 'boning, to create a really full cup:

The yellow along the top was just where I used this effect to trim the edge. This pulled the semi-loose weave tighter, and made for even more fullness in the cups.