one must write a job proposal only after going through the detailed job description and its conditions. if you can fulfill all those conditions and you are confident to make it complete according to client requirement only then proceed with the proposal. writing an effective proposal is not difficult process.
you have to just read requirement and start writing your proposal. always start with greetings which show a respect to client and make you sensible person. write about client requirement in detailed and mention that you have been done this job before (if you have been done this before only) and mention previous work URL or any proof. this will built a belief in the client to work with you.
explain client requirement with the more suitable option that could be tried in this project to make it more feasible and to get maximum output.
show your portfolios as proof of examples, keep in mind that example should be relevant to that job otherwise it could be a bad impression. everything should be in proper manner and structure, no spelling mistake, proper paragraph, and spacing.
after explaining everything you must write that you are available to discuss in detail on this requirement or job as this gives the client a compatibility to work with you. end your proposal with thanks after answering all general questions.