How do I determine exactly which process is draining my battery?
Try the following:


Is the battery drain exclusive to your phone model? Some phones are innately more prone to excessive drain. Find someone with the same phone and do a cross comparison.
Battery capacity degrades over time. From experience, phones over a year do show a significantly lower overall battery life. The battery could be faulty as well.
Check external media. I have seen cases where an external SD card trips the Mediascanner, causing it to run excessively.
Check your running apps. My go to app when dealing with battery drain is BetterBatteryStats. BBS has extremely extensive features to tackle problems such as wakelocks.
Are your running a custom firmware? There are occasions where custom firmwares drain more battery then firmwares issued by the OEMs. 
Utilize the granular permissions to forcefully control rogue apps. Certain apps can silently access location or NFC functionalities.
Try a clean reinstall with stock apps and have it on standby for a full day. Check remaining battery percentage left afterwards. Use it as a control test to check if its the system or user app which is draining the battery.
Enable root and freeze system apps. Google applications and services, while often necessary, takes a good chunk of battery. Identify what you do not need and freeze them. Be warned, certain dependencies might stop your phone from booting.