An example siesta2bgw calculation is provided in examples/DFT/benzene. 
Please see this example for usage and example siesta and denchar input
files.

The SIESTA wrapper reads lattice vectors, atomic species and
positions from SystemLabel.XV file, k-points from SystemLabel.KP
file, eigenvalues from SystemLabel.EIG file, wavefunctions in R-space
from SystemLabel{.K$k}.WF$n{.UP|.DOWN}{.REAL|.IMAG}.cube file, symmetry
group, kinetic energy cutoffs for wavefunctions and charge density,
k-grid dimensions and offsets from the input file, generates reciprocal
lattice vectors, rotation matrices, fractional translations, FFT grid and
G-vectors, performs FFT from R-space to G-space, and writes the selected
range of bands to the output WFN file. Make sure to set up the real
space grid in SIESTA/DENCHAR utility same as FFT grid. If the two grids
differ the SIESTA wrapper will return an error. The output WFN file
can be used as an auxiliary wavefunction for the SAPO code.

A full GW/BSE calculation with SIESTA wavefunctions also requires RHO and 
VXC files. These files can be generated from SystemLabel.RHO{.UP|.DN}.cube, 
SystemLabel.VT{.UP|.DN}.cube and SystemLabel.VH.cube files obtained with 
SIESTA/GRID2CUBE utility on SIESTA real space grid. Make sure SIESTA real 
space grid is equivalent to FFT grid by setting parameter MeshCutoff 
in SIESTA input file same as kinetic energy cutoff for charge density.

Input is read from file siesta2bgw.inp:

&input_siesta2bgw
   systemlabel      = 'ammonia'
   wfng_output_file = 'wfng.lo'
   rhog_output_file = ''
   vxcg_output_file = ''
   ecutwfn          = 60.0
   ecutrho          = 240.0
   wfng_nk1         = 1
   wfng_nk2         = 1
   wfng_nk3         = 1
   wfng_dk1         = 0.0
   wfng_dk2         = 0.0
   wfng_dk3         = 0.0
   wfng_ref_flag    = .true.
   wfng_ref_kpoint  = 1
   wfng_ref_spin    = 1
   wfng_ref_band    = 4
   wfng_ref_energy  = -5.87
   wfng_band_flag   = .true.
   wfng_band_min    = 5
   wfng_band_max    = 28
   wfng_energy_flag = .false.
   wfng_energy_min  = 0.0
   wfng_energy_max  = 0.0
   wfng_gamma_real  = .true.
/

Here, lattice vectors, k-points, eigenvalues and wavefunctions in R-space 
are read from ammonia.XV, ammonia.KP, ammonia.EIG and ammonia.WF$n.cube 
files, respectively. The kinetic-energy cutoffs for 
wavefunction and charge density are set to 60 and 240 Ry, respectively. 
The k-grid is set to the Gamma-point. The SIESTA eigenvalues are shifted 
in energy so that the HOMO state (1st k-point, 1st spin, 4th band) appears 
at -5.87 eV. The real parts of resonant SIESTA wavefunctions (from 5th to 
28th band) are read from Gaussian Cube files and the imaginary parts are 
set to zero (wfng_gamma_real), the wavefunctions are brought from R-space 
to G-space and written to wfng.lo file. The SIESTA charge density and 
exchange-correlation potential files are not generated.

