do you have any guidelines on what the organisers are looking for? Often there is some sort of information package which can be obtained before you write the proposal.

One really important thing to remember is to look at it from the organisers' point of view in terms of the benefits you can offer them, e.g. it will bring in money through sales, and/or lots of happy customers (customer satisfaction) through learning new skills and enjoyment, and/or it will bring prestige to the festival or great publicity etc etc.

To develop the list of benefits think about your previous workshops or events and work out what the benefits were from them. What sort of feedback have you had from other workshop participants or concert goers or organisers.

Also, your credibility can be important, so some sort of "credentials" may need to be included. By that I mean, maybe a list of the other events you have conducted or been a part of, the other festivals you have performed at, your prowess as a musician e.g. how long you have been singing and/or playing instruments, a list of your recordings, video tapes, books or publications, or media coverage you have received.   

Once you have the guidelines, or have spoken to someone who knows what they are looking for it is just a matter of putting it all down in writing and seeing if it flies.

Good luck
Helen