A unique capability of NICMOS is the grism mode, which permits slitless spectra at low resolution. Typically, a direct image is taken in conjunction with grism images for the wavelength calibration. Two IDL programs to extract spectra from such data have been developed at the Space Telescope - European Coordinating Facility . One of it, Calnic C, produces spectra from all objects its detects in an non-interactive way. The other one, NICMOSlook , is a fully interactive widget-driven program. Both programs share the algorithms for the basic spectrum extraction. In addition, Calnic C has as an additional feature the built-in capability to use a sophisticated object detection program. On the other hand, NICMOSlook has additional capabilities to correct the spectra for effects which require interactive intervention. This manual describes the algorithms used by both programs. In addition, it gives the details on how to run Calnic C. A separate manual is available to describe the non-interactive version of the program. The capabilities of Calnic C are the following. Objects are identified on a direct image and classified as stars or galaxies using a neural network approach implemented as the SExtractor program (Bertin & Arnouts, 1996, A & A Suppl. 117, 393). The position of the object are used to extract spectra from a grism image of the same region. The wavelength calibration of the extracted spectra are performed using the position of the objects as determined by the SExtractor program as the zero point and parameterized dispersion relations. After extraction of the spectra, they they are corrected for the wavelength dependence of the quantum efficiency of the detector. The flux scale is then computed using the standard NICMOS flux calibration data. The extracted spectra are corrected for contamination from nearby objects. Subsequently, the extracted spectra are automatically searched for emission and absorption lines. In addition, the continuum emission is automatically determined. The final data products are binary FITS tables with the spectra, error estimates, object parameters derived from the direct imaging and details of the spectrum extraction process as well as postscript files with plots of the individual spectra.