What should I do to plan my maternity leave?

Plan in Advance. Talk to your boss or HR department at the beginning of your pregnancy so that you are not hiding anything. Make a tentative plan in your mind and if something unexpected happens your doctor will help you with it. Make sure you have all the necessary paperwork to your doctor on time, don't wait until the last minute. Once everything is set you can relax and be excited as the time gets close!
Plot future. Think about how happy you will be spending quality time with your new born, while you are not working! plan your daily routine and get your life in order as you realize all the joys of having a new person in your life. Good luck.