Most students fail to follow timetables for the same few reasons, and once you fix those, sticking becomes far easier. The usual mistakes are making the timetable too ambitious, packing every minute with no breaks, being too vague, and treating one slip as total failure.
Build a realistic timetable, not an ideal one. Plan for the student you actually are, with normal energy, breaks, meals, and some free time, not a machine that studies eight hours straight. Leave gaps and buffer time, because the first time real life makes you run late, an over packed timetable collapses and you give up on the whole thing. Be specific about tasks too. A slot that says study is easy to ignore, while a slot that says solve ten algebra problems tells you exactly what to do.
Most importantly, expect to miss slots sometimes and plan to recover instead of quitting. Missing one session is normal, it is not proof that you are undisciplined. The students who succeed are not the ones who never slip, they are the ones who simply continue after a slip instead of throwing the timetable away. Start small, maybe a light timetable you can definitely follow, and build it up as the habit forms. A simple routine you keep for a month beats a perfect one you abandon in three days.
Example: If you miss your evening maths slot, do not scrap the whole timetable. Just move on to the next slot, or shift maths to the next open gap and continue.
One practical tip: Make your first timetable almost too easy. Success builds the habit, and once following it feels natural, you can slowly add more.