On your profile page, click on the "Add a new blog post" link under your profile picture. On the next screen, you can give your entry a title and, under "Post" just start writing. There are also some formatting tools you can use to make text bold or italicized. When you are finished, you can preview the entry by clicking on the "Preview" button. When you are happy with what you have written, click on the "Add a new blog post" button below the text box. Your entry is now visible in the blog section on your profile.