How do you compose a 24 measure of music

It depends on what style, mood, and instrument you compose for and what you use to compose it. You first start out with the clef; treble, tenor, alto, bass, ect. Just remember that each instrument reads all of these clefs a little different. For example: Flute reads treble clef in concert pitch while French horn does not. Cello reads bass clef in concert pitch while bass reads an octave higher than the note that is actually produced. Next you chose the key you want to write in, major, minor, or diatonic. Last but not least you choose a time signature such as 3/4, 4/4 (also known as C or common time), 6/8 12/8 ect. Then you start writing, and keep in mind that the first and last note set the key you are in.