Start by piping the base of the tree. Using a decorating bag fitted with Tip 16 (or any small star tip), hold the bag with the back end pointing to you and the tip just slightly above the surface.
Squeeze the bag to form a star. Use light pressure to form a small star; add more pressure to make a bigger star.
Stop squeezing before pulling the tip away.
After completing the base, pipe a shorter row of stars right above. Stagger as you pipe, so that the new row of stars is piped between the previous row of stars. This will create a fuller, filled-in look.