Read through the music sheet and determine each note used. Transfer these note names to a sheet of paper. Separate the notes in the same way they are separated on the sheet music, using bar lines on the paper. If the notes you have on the sheet music are C-A-B-G, write those notes in letter form on the piece of paper to which you're transferring the numbered notes.
Write a number next to each note you transfer to another piece of paper to indicate the octave in which the written note is played. In the example above, if C-A-B-G are played in the third octave (pianos are typically used for octave context reference), write the number 3 beside each of the letters. It will look like this: C3-A3-B-3 and G-3.
Write a number below each of the letter/number combinations to indicate how long a note will be held out. This is the rhythm of the song. Common time is 4/4, which means there are four beats to a measure of music. You can use any number below the letter/number combinations to equal your four beats. For instance, if the C-3 note will be held for 2 beats, indicate this by writing the number 2 below the C-3. If the rest of the notes are held for one beat each (to equal 4), place a number 1 beneath each.