Tell HRTM/HRTL/ETL-HR that you need an LOA. They'll ask why. Different form for personal LOA and educational LOA.
They'll ask you the day you can last work, and when you'll be back. Cross some Ts and dot some Is, you'll be good to go! Stay in touch with your HR if you're coming back early, late, and let them know three weeks before you come back so you can be on the schedule for sure.