a) having an index.ndx file specifying the QM atoms (see example index.ndx file). You can create a usual Gromacs index.ndx file and then append to it the QM group.
b) specifying other QM information like plane wave cutoff, qmbox size etc in the grompp setup (for the mdp file).
c) having a CPMD input file template CPMD_inp.tmpl where essential keywords for CPMD run need to be mentioned. INTERFACE GMX is essential for QMMM; it ensures a single-point calculation inside CPMD each time it is invoked. Inside the interface, all the QM & MM atoms are translated in such a way that the QM system be at the center of the QM box. Thus the keyword CENTER MOLECULE OFF is required to avoid any further movements of the QM atoms.
Right now the ODIIS minimizer and PCG minimizer (including PCG MINIMIZE) are allowed to be used inside CPMD. There also is a hybrid scheme where for the MD first step it will use the PCG MINIMIZE but for all subsequent steps it will use the faster ODIIS minimizer.). Other sections of CPMD input structures need to be kept as usual though the final values for the CELL size and CUTOFF will be those provided by you in the mdp file.