The object of plating is to modify the original colour, either by changing the tone or the intensity. This will, for example, darken a piece of glass where it would otherwise be to bright; or it will modify the colour to better blend with the surrounding pieces.
A further use of plating is in conservation, where the additional detail is placed on a separate piece of glass and placed in front or back of the original.
In leading, you normally use high heart lead. This is lead with a heart of 7mm or 10mm instead of the usual 5mm. Other heights are available, of course. The 7mm heart will accommodate two 3mm pieces, but if you are using thick hand made glass, you may require the 10mm high heart.
Comparing the Arrangement
Try the glass combination with each piece on top. Often there is a difference in tone or texture. Choose the one that suits your composition best.
Before finally fixing the glass together, make sure they are very clean as there will be no opportunity to clean the inside again. Try to avoid finger prints on the insides while you do further work with the glass.
Make sure the glass fits the cartoon lines. You will be sealing the two pieces of glass together, so there is no opportunity to change the shape later. There are a variety of traditional methods of sealing the glass, but the easiest modern approach is to copper foil the edges to ensure that no cement creeps between the pieces.
You then fit the glass into the came as for thiner pieces. Where you have a combination of heart heights, you can simply slip the ends of the lower heart cames inside the leaves of the high heart leads. The differences in height are small enough that no special support is needed for the thinner glass unless you feel better with the single layers of glass supported above the work surface.