How do you decide which textual variant belongs in the Bible?

There is no exact method; you use logical reasoning. If fifty manuscripts contain a Greek phrase and only one omits it, logic says the odds it is original are about fifty to one, because the lone exception may simply be a copyist who fell asleep, as happens after hours of staring at manuscripts. We have errors in our Greek copies because fallible people made them, and there are about fifty-seven types of copying mistakes. The Jews were reportedly so strict that a superior would discard an entire copy at the first error, and the medieval monks who copied the Greek were extraordinarily dedicated because they believed it was the word of God. That is why only about one percent of our five thousand Greek manuscripts show discrepancies, and those are minor, a letter or a tense, with perhaps three or four larger ones like John 8 or Mark 16, none of which undermines the Bible's integrity. There are about half a dozen theories of textual criticism for deciding the correct reading, and I have studied them all; every one has flaws, which is what we expect, since man can come close but not achieve perfection. Still, of all the copied books in history, the Bible has by far the fewest errors, far fewer than copies of Homer or Shakespeare, so you can be confident you hold the word of God.