Declare - to make known, to announce officially, proclaim.
Decree - a formal and authoritative order, especially one having the force of law, to command, Theology - one of the eternal purposes of God, by which events are foreordained.
What's the difference? The level of authority. Declaring, I align myself with God's Word and speak truth aloud. Decreeing is exercising spiritual authority to establish God's will or commands for circumstances to change.
In the declaring I speak His truth, in decreeing I exercise the authority given by Jesus to speak into situations in the earth. Both require a willing voice.
"You will decree a word, and it will stand for you, and upon your paths light will shine." Job 22:28
Decree a word. To exercise a decree of any word is useless and will not change life's situations. To decree God's Word, can change everything. I think of the Lord's Prayer, "Your kingdom come, your will be done on earth as it is in heaven." I decree that often, speaking the Lord's Words back to the Father. His kingdom is coming, see Revelation 21:1-2.
May I only declare what God Almighty decrees. May I declare His glory among the nations, His marvelous deeds among all people.