No matter what project we're talking about, the hardest thing is figuring out exactly how long things will take. There are tools that pros developed that can help you determine what tasks need to be done at what time, and what has to go happen for one task before a second task can start. I'm not sure a short journal can cover that all, but I can talk about something else. Scheduling work is a major challenge in managing a project.
A professional project is slightly easier because you know how many hours you've got per person. Amateur projects are harder, because you can't be sure. All the work is done during free time, and that varies.
Make sure you keep in mind exams, studying, projects, part-time work, vacations. Assume no work will be done in those periods- it's much safer than assuming "well, they can maybe work at half-speed then".
Ask everyone on the team what sort of things will be coming up, and account for them.
Unexpected emergencies may crop up, too. Allow some slack time for them.
In the end, the most important thing for an amateur project is flexibility. Timelines are at best a guideline.