Q: What rule hath God given for our direction in prayer?
A: The whole word of God is of use to direct us in prayer;1 but the special rule of direction is that form of prayer which Christ taught his disciples, commonly called The Lord's Prayer.2

1 John 5:14. And this is the confidence that we have in him, that, if we ask any thing according to his will, he heareth us.
Matthew 6:9-13. After this manner therefore pray ye: Our Father which art in heaven, Hallowed be thy name. Thy kingdom come. Thy will be done in earth, as it is in heaven. Give us this day our daily bread And forgive us our debts, as we forgive our debtors. And lead us not into temptation, but deliver us from evil: For thine is the kingdom, and the power, and the glory, for ever. Amen.
compared with Luke 11:2-4. And he said unto them, When ye pray, say, Our Father which art in heaven, Hallowed be thy name. Thy kingdom come. Thy will be done, as in heaven, so in earth. Give us day by day our daily bread. And forgive us our sins; for we also forgive every one that is indebted to us. And lead us not into temptation; but deliver us from evil.

Wesminster Project - Thomas Watson on why the Lord's Prayer should be held in such esteem