While some people may be able to prepare themselves in a year to apply and start in a PhD program, it is better to plan and start ahead, especially your major is not Economics or mathematics. It is often times becasue your quantitative skills are very important to show schools that you are good enough to go through all the coursework and research as a PhD student. While the below is not necessary a perfect timeline, it will give you some idea for your planning to apply to finance PhD programs.
2+ Years Prior to Applications
Summer of Application Year
Fall of Application Year
Spring of Application Year