Our research shows you’re more likely to have success with a custom-tailored proposal. Include how you meet each of the client's and project's needs and requests. This includes location, skills, language & communication abilities, past experience, education - and anything else the client notes that is important to them. If relevant, include links or attachments to work samples. This shows the client you are capable of completing their project. You can also check this article, here you will find how to make your cover letter even more successful.