Here’s how he begins:
Textual critics are the scholars who study the manuscript evidence for the New Testament and determine which readings among the various manuscripts are likely to be the original (see previous post for background). Although the vast majority of the variants are simple spelling or word order errors made by copyists, there are some variants in the manuscripts that are more significant.
Textual critics use some basic criteria to help determine which readings are most likely the original and which variants were added or modified by copyists.
via How Do Textual Critics Choose Among New Testament Manuscript Variants? at The Poached Egg (Ratio Christi, Greg West).