Think about your book idea. Try to work through the material that you will be covering if it's a nonfiction title. Consider all your possible characters and the point of your story, as well as the key points. Make sure you know exactly what your book will be about.
Make a rough outline of your ideas by writing down a list of bullet points. Write down anything you think is vitally important for people to know. Sort through the bullet points and pick out the seven or eight that are most important.
Open a new document on a word-processing program. Look at your selected bullet points. Type them on your document in chronological order (for fiction) or logical order (for nonfiction). Write a 300-to-400-word abstract that combines all your bullet points in logical fashion. Read it over several times and make changes as needed.
Have someone else read your abstract and reiterate back to you what your book is about, what makes it different and key features it has. Based on their answers, make changes. If the person has no idea what your book is about, more work is necessary. Ask for their feedback.
Proofread your abstract and check for spelling and grammatical errors. Have someone read it again and get feedback one more time to make sure it conveys what your book is about. That's it! You just wrote your abstract!