Buyers can receive many many Proposals and can't always review them all, to make yours catch the Buyers eye you should firstly read the Job description so that you know what the Buyer is looking for.
Once you have this information you can start to write your Proposal.
Make it relevant to the Job, do not overwrite, Buyers can be very busy people and do not have time to sift through a mountain of text to get to the desired skills. Consider your Budget. Be honest, don't say you have skills or experience that you don't have.
Once you have completed your Proposal you can offer further information to support your claim via the message board on PPH.