Understand the prophecy
To understand the gift of prophecy and how it works is very crucial, so as to use it accurately. Prophecy is often an operation in an spoken of faith. A spoken of faith is declaration of words that will come to pass, or words that you want it to come to pass.
The gift of prophecy have to work through revelation, it's not just through your utterance of faith. The words you speak may not be consistent with God’s revelation. This shows that you can’t just stay with the fore told part of prophecy, you need the revelatory part as well.
You can not use that level of merely declaring words of faith to change the course of a nation, a city, a family, or a person’s life. You would have to know the will of God, and the direction of the Holy Spirit, to speak words that are in agreement with the Spirit of God in order to effect the kind of change that is consistent with God's will.