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The COS Legacy Archive Spectroscopic SurveY (CLASSY) is designed to providethe community with a spectral atlas of 45 nearby star-forming galaxies whichwere chosen to cover similar properties as those seen at high-z (z>6). Theprime high level science product of CLASSY is accurately coadded UV spectra,ranging from ~1000-2000A, derived from a combination of archival and new dataobtained with HST's Cosmic Origins Spectrograph (COS). This paper details themulti-stage technical processes of creating this prime data product, and themethodologies involved in extracting, reducing, aligning, and coaddingfar-ultraviolet (FUV) and near-ultraviolet (NUV) spectra. We provide guidelineson how to successfully utilize COS observations of extended sources, despiteCOS being optimized for point sources, and best-practice recommendations forthe coaddition of UV spectra in general. Moreover, we discuss the effects ofour reduction and coaddition techniques in the scientific application of theCLASSY data. In particular, we find that accurately accounting for fluxcalibration offsets can affect the derived properties of the stellarpopulations, while customized extractions of NUV spectra for extended sourcesare essential for correctly diagnosing the metallicity of galaxies via CIII]nebular emission. Despite changes in spectral resolution of up to ~25% betweenindividual datasets (due to changes in the COS line spread function), noadverse affects were observed on the difference in velocity width and outflowvelocities of isolated absorption lines when measured in the final combineddata products, owing in-part to our signal-to-noise regime of S/N<20.

CLASSY II: A technical Overview of the COS Legacy Archive Spectroscopic SurveY