Print templates. Cut out spiky spine template. Using disappearing-ink pen, trace the spikes of the spine onto felt. Cut around the spikes, leaving them attached to the felt as shown.
Cut through the slits in the slit template. Place it on the shirt, and trace lines through the slits. Snip out the slits on the shirt. Place the felt strip inside the shirt. Pull the spikes through the slits so they stand up.