Use alarms and don't ignore them, ever. Set the alarms to go off at the time when you want to start setting up for the next part of your day; e.g. getting ready for bed instead of lights-out time, setting up your workspace for the day instead of time to be fully productive, checking if you're hungry instead of lunchtime, &c. You can set as many labeled alarms as you like on your phone and many watches, and you can schedule them to repeat regularly. If you don't want to disturb people around you, set the alarm to vibrate and keep the device on your person. (A smart watch is exceptionally useful for this.)
If you need additional alarms to remind you to actually get started, set those too. I prefer to use just one alarm and let setup naturally flow into the intended activity, but do what you have to do to keep your day moving the way you want it to. Remember: never ignore your alarm. If you didn't want to do the thing, you shouldn't set the alarm in the first place! If you can't actually start "getting ready for bed" (or whatever) when the alarm goes off, acknowledge the alarm and begin moving toward that goal. The setup phase can start with whatever you're doing right now and ends when you're ready to do the next thing, but it's important to get that process moving!
Revisit your alarms as often as you need to to make sure you're cueing the right habits/systems. This will be more frequently at first, but as you settle in to the routine you want you can review less often. And don't be afraid to make changes if life takes an unexpected turn. If you think you might need an alarm later, turn it off instead of deleting it. That way you can just turn it back on again or reconsider deletion when you're more sure.
Keep alarms only for your normal schedule. For events that occur irregularly, infrequently, or just once (e.g. next month's game day, maintenance schedules, dentist appointments) schedule calendar reminders with appropriate lead times instead of setting alarms. This kind of reminder will vanish from your active systems automatically after it has fired, and you won't clutter up your alarm cluster with dead items.
If a consistent bedtime is a problem for you, try working from the other end. Getting up at the same time every day means that if you didn't get enough sleep you will be more tired in the evening and it will be easier to go to bed when you want. You can use this to explore how much sleep is optimal for your body and establish a bedtime you will want to keep.