Quick answer: swearing. But not the best answer.
Taking God’s name in vain means to use God to endorse words or actions for which He as no support. Here are some examples:
1. “God damn you – go to hell.” God damns no one. People choose hell for themselves by their life behavior and beliefs. John 3:17 (we read verse 16 then ignore 17) “For God sent not his Son into the world to condemn the world; but that the world through him might be saved.”
2. Stating as God’s will something that is clearly false.
a. There is no New Testament proof that Jews, as a group, go to hell. They are God’s people. The book of Romans chapters 9-11 clearly tells Christians to mind their own business. Besides, Jews have Muslim enemies enough without others butting in. Jews are God’s business and it is our job to pray for their nation and their safety.
b. The Supreme Court heard a case involving the Westboro Baptist Church which was hassling funerals of soldiers with yelling and signs, like “God hates fags” “God hates abortion” “God hates America.” It is true God hates sin, but sinners he accepts. The plan of salvation is not complicated. The members of this group have substituted their conclusions for what God is saying and doing. There is no Biblical link between dead soldiers and what God hates or does not hate.
3. Final example. “God told me you are to marry ____.” Let God do his own talking please. Can God tell you whom to marry? Sure, but He tells the participants not others. To butt into a marriage is called adultery, even if it is only in your mind or your words. Find something else for your mind to do. And if you are a man or woman who has been told this, back off and start thinking seriously about what is going on. God wants to work through you for His glory. He can lead you the whole way.