Write a few songs so that you can have some in your portfolio. If you want to concentrate on one genre, you can do that, but if you want to be more successful at being a music ghostwriter, you might want to make some songs in more than one genre.
Either perform these songs for yourself, or hire someone to make a recording of them.
Put together a portfolio of songs that you have written. Make sure that you include snippets of these songs as well as the actual sheet music for the songs.
Now that you have a portfolio, you need to shop around for clients. A good music ghostwriter will have several different performers, acts or bands that they are writing songs for. The best way to get clients is to contact local musicians who sound like they might enjoy your songs. In the past, going to local shows was the best way to do this, but currently you can use social networking sites to do this as well.
Contact members of bands or music acts and let them know that you are interested in ghostwriting songs for them. Show them your portfolio and ask if they would like you to write songs for them.
Each time you ghostwrite a song, be sure that you have a contract. It can be up to you or to the band whether you will be paid a one-time fee for the song, or whether you will get a share of the profits from the song. Either way, you must be sure that a contract is legally binding and is signed by all parties before you give them the song.