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You think of all tasks you can do at the moment, things where you are not blocked by anything. You prioritize them, say on urgency or on what you personally feel like working on if your work permits freedom. This takes 60 seconds at most because of course you are anal about keeping a sane, up to date diary/to do list/kanban board.
You start working. You don't check your email, slack, messages, whatever. Let them call you on your phone if the world burns - but even that you let go to voicemail on the first call. You just start working without delay. And you keep going. After the first 5-10 minutes of forcing yourself to settle into your chair and ignore your painful wrists, you enter a state of flow.
It'll be like driving eventually. Remind yourself to take a break every two hours, drink something and maybe eat a little. Dehydration is a bitch. But even as you take a break, your mind won't let go completely. You keep this up for 4-8 hours a day, answer some messages and turn off your computer... hopefully.
Be sure to really unwind at the end of the day. Doing this 4-5 days in a row without unwinding will mess up your weekend big time as you just can't go back to baseline