Outline the major points you want to make in your speech. If you're not sure what these are yet, spend some time studying the music about which you want to talk. Think about how you would describe or explain the piece and the links between multiple pieces of music, to a friend who wasn't familiar with it. Write out some notes and observations about the piece, then try to condense these down to the most important points.
Write out your speech in its entirety if you don't plan on improvising any of it, or write out a few key passages and sentences if you do want to work without a script. In either case, start with an outline based on the key points you determined in Step 1. Put these key points in what you think is the best order and fill in background information enough to help people understand these ideas.
Prepare musical examples that you can play during your presentation. For greatest ease, use a computer mp3 player program with a sliding track bar display. Listen to your music and make notes of the times in the tracks that you want to play.
Draft visual support materials to help your listeners understand the structure of the music. This may include photographs, if the music has any visual art influence, line diagrams if you're discussing the structure of a composition, or enlarged samples of sheet music if you're pointing out features related to music theory details. Plan when you're going to introduce these materials in the context of the speech.
Practice your speech to work out timing. Use a stopwatch to time yourself and see if the speech is at the length you want. Add or trim as necessary.
Perform your speech in front of a friend and get feedback. Ask whether the speech was engaging and clear. For best results, practice with someone who has roughly the same level and type of background education on the subject that your final listeners will have.