We've all done it. We spend 3 hours making a beautiful Excel spreadsheet detailing every minute of our day ('8:00 AM - 9:00 AM: Biology, 9:00 AM - 10:00 AM: Physics'). By 9:15 AM on the first day, we are off track, and we give up on the whole timetable.
1. Schedule by Tasks, Not Time
Instead of blocking out '1 Hour of Math', block out 'Complete May/June 2021 Paper 4'. If it takes 45 minutes, great, you have free time. If it takes 1.5 hours, that's fine too. Task-based scheduling focuses on output, not just sitting at a desk.
2. The Buffer Day
Things will go wrong. You will get sick, or a family event will happen. Always leave Sunday completely blank. If you fall behind during the week, Sunday is your catch-up day. If you stay on track, Sunday is your reward (zero studying!).
3. Prioritize Your Weaknesses
It is psychologically comfortable to study what we are already good at. Force yourself to put your weakest subjects in the morning when your willpower is highest.