You cannot achieve deep focus while continually switching back and forth from one task to another.
To perform any task with an optimal state of focus you need to reach a state of flow. You can’t do this just by wishing it so. Or snapping your fingers.
First you need to block out any immediate physical, digital and mental distractions. Then spend around 15 minutes of uninterrupted work on the task to reach a state of flow.
Time will bend, work will become effortless and doubt will evaporate.