First I make a sketch on the cast bead to define the proportions and get the layout in mind.  For reference I use photos of classical robed people from something like a dover clipart book or photos of antique wedgwood vases.

I do my slip trailing on dry porcelain but most people do it on wet pieces.  I prefer doing it dry as it allows me to do more detailed carving and modeling of the slip trailed design resulting in a more detailed piece.  Time is also not a consideration because it is already dry so no need to worry about it drying out.

Next I build up the underlying body shapes that show through the draped gowns.  As this is built up it is frequently smoothed out with a damp brush to insure good adhesion to the base material, prevent cracking, and achieve a smooth surface.

In this photo you can see how the color of the white porcelain and the color of the blue porcelain of the bead are nearly the same.  This increases the difficulty of this method of decoration because it is very difficult to see what you are doing.  Bright light is essential.  Also because the white porcelain is translucent when it is fired some of the blue will show through in the thin areas and it is very difficult to tell how much porcelain is needed to achieve a totally white area and how little is required to give the illusion of transparency and shadows.

Additional slip is added to complete the gown, face, hair, etc.  Again it is frequently smoothed with a damp brush.  A small x-acto knife is used to carve in details such as the eyes and some of the folds in the gown.  Also you can see that the large hole at the bottom of the bead was closed in and decorated with a leaf design for added decoration and to reduce the size of the stringing hole.  The same sort of leaf design was applied around the hole at the top.

This photo shows the bead ready for firing after several hours of decorating time.