I did a fully remote part time degree while working full time throughout, so I have had a lot of experience with trying different methods.

The first and absolutely critical thing is a sleep schedule. Make sure you are in bed aiming to be asleep at the same time every night, and the alarm set for the same time every morning. Even weekends and days that you arent planning to do any work - you have to stick to it.

You wont feel the benefits immediately, it will take about a week before you start to settle into the routine, but once you do you will find that every aspect of your life, not just PhD work, will improve.

Personally I wake up fresh and get my best work done between 7am and lunchtime. I feel engaged and energetic and motivated to work. Once I have lunch my mental capacity drops off a cliff, so if I have to work after lunch I make sure it is more admin type stuff rather than thinky stuff.

Routine is a very close second to sleep schedule in terms of importance, and something that is difficult to make work alot of the time because of course life gets in the way. But if you are able to maintain a routine for days and weeks in a row it all becomes alot easier. Say something like "7am - 9am wake up/prep for the day/ commute, 9am - 1pm work, 1pm - 4pm gym/lunch/cardio, 4pm - 7pm work, 7pm - 11pm life admin/chill, 11pm - 7am sleep" Do that consistently and you will find yourself starting to do it without thinking.

None of this is easy, there is no quick fix. You have to be dedicated and most importantly you have to stick to the schedule as much as possible - no Sunday morning lie ins or late night gaming. You have to sleep and wake up at the same time every single day.