Stencils are available in many sizes and shapes, some designs are simple, and others more intricate.
Most of them for cake use are flexible, which is a help when manoeuvring around corners. Others are hard plastic, mostly small for cupcake tops. There are several ways to use them in cake decoration depending on the effect you wish to create.
Using paste/gel colours
Lay your stencil onto the sugar paste, evenly press down into the paste and secure if possible. Dilute the food colour slightly, and using a brush or sponge, carefully apply the colour sparingly, before removing the stencil.
These may be brushed/painted on, or applied with an aerosol spray can. Push the stencil slightly into the paste so that the paste stands proud. Apply a thin smear of vegetable fat over the surface of the stencil and apply the lustre powder/paint sparingly either with a brush, or spray on. If dusting on, brush off any surplus powder before removing the stencil.
Roll out sugar paste. (This method may also be used on a royal iced surface). Place stencil onto paste/icing and secure. Apply a thin layer of royal icing with a small palette knife (cranked is easiest) over the stencil design. (If colouring the design, leave the stencil in place until dry and paint). Remove stencil carefully and you will be left with a very attractive raised design. This method looks great on a coloured background with a white design.