Buy staff paper, or print it from websites that offer it for free. Check the Resources section for an example of one of these sites.
Add the appropriate clef markings on the left side of the staff lines. Most choral music or piano music has a treble clef on top and a bass clef on bottom. If you're writing in both the treble and bass clefs, you'll also need a brace on the left side that groups the two staffs.
Add the key signature. This shows which sharps and flats are in the music, which indicates the key.
Add the appropriate time signature, which shows how many beats per measure, as well as how many counts certain notes receive.
Write in the notes of your music, being sure to divide the notes into measures (according to the time signature). Remember that the lines of the treble clef are (from bottom) E, G, B, D, and F, and the spaces are F, A, C, and E. The lines of the bass clef are G, B, D, F, A, and the spaces are A, C, E, and G.
Add articulation markings if you wish. This indicates certain styles of playing, including considerations for volume and mood.