I know you're modest. I know you don't like to brag and boast about what you've accomplished. But in the "About the Author" section of your book proposal you've got to overcome your natural inclination to be reticent. Come right out and say all the positive things about yourself and your writing credentials that an agent would say if she were representing you.
Not your style? Don't worry, I'm going to tell you an insider secret all published authors have mastered which will make you look good without making your feel guilty. Examine any author blurb and you'll see what I mean. They always write about themselves in the third person. That's the secret! So, instead of saying, "I published an article in such and such magazine" you say "[Fill in your name here] published an article in such and such magazine." Trust me, once you start referring to yourself in the third person it will be easy and fun to write your "About the Author" section.
By William Cane
What if you have no previous publishing credits?
Even if you have no previous publishing credits, it's still possible to get a book contract. Just focus on whatever positive credentials you do have. For example, if you're a scientist focus on your education and work experience. If you're a professor, focus on the courses you've taught and the research you've done. If you're a housewife, focus on the unusual nature of your situation. If any newspapers or magazines have expressed an interest in your story, mention that. Publishers know that everyone has to start somewhere, and while they prefer working with established authors they're also eager to discover new talent.
Edgar Rice Burroughs was a master publicist. Even when he submitted his first manuscript he didn't hesitate to tell the editor that he was great. Don't overdo it, of course, but if you write in the third person you'll strike just the right tone and you'll get across the message that has to be included with your book proposal, namely that you're the best person in the world to write this book.