Concisely, write what you can offer to the client and how you plan to go about it. Add a short explanation of your approach; for example, “should we proceed with the contract, I shall <1>, <2>, and <3>”.
Don’t (1) beg for the job, (2) declare how good you are, (3) give out too much information, (4) say stuff like you will do the work on time - there is no point of saying that. If you are applying to the job, then it is understood that you will finish the work within the deadline.
In short, maintain your dignity, do not sound desperate, and you will find your clients. By the way, it is a waiting game when it comes to freelancing; the more number of proper bids you send out, the higher your chances of receiving clients.