How does the canon guide us in regards to textual variants (e.g. the Comma Johanneum)?
The point about the canon and text is to show that the church has an important role as a witness to the text just as it does to the canon. On that basis, if you were to ask how the church can guide us, I could make sense of your question. As it stands in the context of this discussion, to ask how the canon can guide us makes no sense.
To give an example of the church witnessing to the text as well as the canon, consider the disputed ending of Mark.