Judging is giving one’s opinion about a matter. The words come from the heart and mouth of the one speaking and not from a law written with authority.
For example, texting and driving is wrong and one isn’t judging if he said that to another person. The law which is a higher authority has considered and condemned texting behind the wheel. So one person warning another is simply reminding people of what the law has said (Jonah 1:1-2, 3:1-4).
The same is true when it comes to the word of God. Many of the deeds done by man are sinful (1 John 3:4). Those who study the scriptures recognize this and tell others what is wrong. They aren’t judging! God has done the judging! God has said that something is a sin that it is a violation of His law. So the one speaking is simply informing the offenders what the Lord has revealed (Genesis 39:7-10).
So giving our opinion on a matter: that we will never do (Matthew 7:1-2).
Telling others what God has revealed in His word: that we will do (2 Timothy 4:1-5).