Routine is a luxury. It requires that you have considerable control over your circumstances. If you are early in your career, still living with your family, or still in school, you probably don't have that kind of control.
And even when you've found your footing, routine can go away. I recently moved into a new apartment and it broke most of my routines. That's a benign example. There are so many other events that can come between you and your control of your circumstances.
So be consistent instead. Having routines is huge benefit to consistency, but it's not a requirement.
Think about an electron cloud. We call it a “cloud” because it's so difficult to pinpoint the exact location of any one electron. They move around so fast and without any obvious patterns. There is no routine to their movements.
But when you zoom out, the behavior of electrons becomes part of electricity. At that point, the broad patterns and characteristics of electrons are so consistent that we can use them to power our machines.
Likewise, you can be consistent with your art even if your routines are messy or nonexistent. You can be as random as an electron and still generate electricity.
One way is to make a deal with yourself. “I will write 300 words every day” or “I will write for 7 hours total every week.” Keep in mind here that the humble electron does not exert any massive amount of willpower to power your lamp. Likewise, you need not make a deal with yourself that feels like lifting a mountain every day.
In fact, it should be a number that you can wrap your head around. A number that feels natural. At least at first. You can increase it later, but only when you've reached consistency with the current number.
And remember that consistency need not only be a number. You can be consistent in how much you care. You can be consistent in how honest you are. You can be consistent in getting better.