For this method you will need a way of playing music (CD or mp3 player).
Pick a piece of music. Any piece you like. Using your music player, listen very, very carefully to the music – try to pick up the pattern of the notes and rhythms. Listen to this section of music several more times until you can sing it note for note from memory.
Once you can do this take out a blank piece of paper and draw out a ‘map’ of the music. How many bars are in each section? Which instruments? What does the melody do? Any repeats? etc. Doing this will help to know where you are in the piece.
When you’ve done this it’s time to write some actual music. Setup a piece of manuscript paper with the clef, time signature and key signature you need. Add any instruments you need. Begin to try and write down the music. Trial and error is OK, you may want to check what you have written on an instrument – that’s OK as long as you try to do most of the work by ear.
Complete as much detail of the music as you can until you have a ‘Carbon Copy’.
You may find that this is a slow process at first but make sure you persevere. The more you do this the easier it will get!