PRAY! In order to speak for God, you should speak to God first. Present your mind & spirit to Him in order to fully receive whatever Scriptures & revelations He has for you to share with the congregation. Prayer is not just talking, but it’s listening to His voice also. Kee your pen & paper nearby for those profound insights that come from seeking the Lord. There is no mandated time frame, but ensure that you are in a place to totally meditate on His Word for the people.
STUDY! When you are given a subject, theme or Scripture in your mind & spirit, do not take that for granted. Study that subject or theme in your study Bible for references. Use the dictionary to find out the different meanings of particular words in that Scripture. Dig into the hisory surrounding that verse of Scripture.
ORGANIZE! Once you have all of these notes & one-liners, follow through with organizing your sermon in a continuous thought. Some minister may feel comfortable with word-for-word paragraphs to speak from. Others may only need an outline to keep them in line with a continuous thought. I have seen some use just keywords to remind them of certain revelations & ideas. There are also some who have studied the Word of God to the point that there is no need for any type of script. Whichever is comfortable for you, master & perfect that so you are confident with what God has given you to give His people.