I try to use the same routine daily with my favorite app for 20 mins. Make sure it's something easy that u can do that won't discourage u from doing it again. Do it at the same time every day to from the habit. It's ok if u miss a day every once in a while, but using an app makes it easier to keep track. The app will allow u to set alarms/notifications, so u can stay on track. U could do just one easy activity that u do anyway, like walking, for 5 mins every day. The point is to FORM habits so ur brain is being REprogrammed for ur new goals. Drink lots of water too!
I vary it! I usually do what feels the best in that moment (maybe the day before I had a certain training so I need to stretch more certain muscles, or stuff like that). Plus, I think that variety is the key to keep things interesting and not boring, because if they become boring, then it's more probable that we stop doing them
Yes, I follow a workout app which keeps things fresh by doing a different set of stretches each day. I also try to choose stretches according to what muscle groups I've worked on recently