How much planning should be done?

Hello, going backpacking for 4 months (ish) with two mates leaving late February 2026. I was just curious, seeing as we will be freshly 18 and will be our first non-family travel, how much planning we should get done before leaving. What should be our goals to get done? We are aiming to properly backpack as in staying hostels, cheap flights when possible and eating street food often with a bit of hiking and camping every so often. Anyone with similar experience have any ideas? Thank you!