The work of Irish ceramics artist John ffrench provides a wonderful starting point/stimulus for clay work at Christmas. His ceramic christmas trees demonstate the concept of woking with clay on a flat surface - in the curriculum this is referred to as "working with clay plaque techniques" and "using coils and pellets for decation purposes".
Roll clay out, preferably on a wooden surface to prevent sticking, using a rolling pin.
Dont make the clay slab too thin, as it has to stand later.
Use a knife/clay tool to create a tree shape (children might create a card shape to help with this).
Design and cut out small shapes to act as decorations for the tree.
Stick these to the tree using slip - make slip by dissolving clay in water to make a thick clay paste.
Make a clay base for your tree.
When dry, add colour with poster paints mixed with PVA glue.