First know that you are abiding in Jesus and keeping His commandments. If you can hear God, that is a sure sign that there is no sin separating you from Him. Then know who you are in Jesus and command the demon to leave in the name of Jesus. Otherwise, the demon will just laugh at you, and may even attack you. Fake Christians have no authority.
Once the demon is gone, make sure that the person knows Who freed them, and let them know they must be filled with the Holy Spirit, otherwise 7 other demons more powerful than the first one will repossess them.