This is step-by-step example how to calculate phonon dispertion with CASTEP.
1. Generate supercell based on structrural parameters given in unitcell.cell
file by running:
% phonopy -d --dim="2 2 2" --castep -c unitcell.cell

In this example file unitcell.cell contains initial spin values in  positions_frac BLOCK.
As well as the value of the U parameter in LDA+U approximation.

2. Use make_displ_dirs.sh bash script to make directories with input files needed to
calculate FORCES for supercells.
% make_displ_dirs.sh supercell.param tail.cell

Please note file supercell.param is the standart CASTEP file with paremeters (convergence
criteria, Ecut, functional name etc.). The file with name tail.cell contains k-mesh
description,  U parameter value, symmetry operation E, and name of pseudopotential.

3. Run CASTEP calculation in displ-001 directory

4. Generate FORCE_SETS file by running:
% phonopy --castep -f displ-001/supercell.castep displ-002/supercell.castep

5. Finally plot phonon bandstructure by running the following command:
% phonopy --castep -c unitcell.cell -p band.conf
