  We present an in-depth, high-resolution spectroscopic analysis of the M dwarf
K2-18 that hosts a sub-Neptune exoplanet in its habitable zone. We show our
technique to accurately normalize the observed spectrum, which is crucial for a
proper spectral fitting. We also introduce a new automatic, line-by-line
model-fitting code, AutoSpecFit, that performs an iterative ${\chi}^{2}$
minimization process to measure individual elemental abundances of cool dwarfs.
We apply this code to the star K2-18, and measure the abundance of 10 elements
- C, O, Na, Mg, Al, K, Ca, Sc, Ti, and Fe. We find these abundances moderately
supersolar, except for Fe with a slightly subsolar abundance. The accuracy of
the inferred abundances is limited by the systematic errors due to uncertain
stellar parameters. We also derive the abundance ratios associated with several
planet-building elements such as Al/Mg, Ca/Mg, Fe/Mg, and (a solar-like)
C/O=0.568 $\pm$ 0.026, which can be used to constrain the chemical composition
and the formation location of the exoplanet. On the other hand, the planet
K2-18 b has attracted considerable interest, given the JWST measurements of its
atmospheric composition. Early JWST studies reveal an unusual chemistry for the
atmosphere of this planet, which is unlikely to be driven by formation in a
disk of unusual composition. The comparison between the chemical abundances of
K2-18 b from future JWST analyses and those of the host star can provide
fundamental insights into the formation of this planetary system.
