Some people outline and some don’t. Neither way is wrong. Some people do both, depending on the book they are writing and how it comes into their head. If you do outline your book before diving into the first draft, make sure your outline stays an outline and doesn’t become so detailed that you feel like you have written the book before you even start the book.
If you dive too deep into your outline, you may loose your motivation to actually write the book from start to finish. You will feel like you have already written it and it’s no longer interesting or fun. When you loose the fun and interest in the story and characters, it is very difficult to keep the tension you need to keep your readers turning pages.
How do you know if you are taking your outline too far? Your outline is becoming very lengthy and very detailed. You are not just making some notes with a few details and maybe a line or two of dialog, you are fleshing out entire scenes and full on conversations. You are describing every detail of a room and the clothing or other things present. You delve deep into the thoughts and feelings of a character.
An outline is just that, an outline. It gives you a sense of direction as you move from scene to scene and from plot point to plot point. It is not your manuscript. It shouldn’t look like your manuscript. A reader should not want to read your outline, although other authors may want to.