Medium has the article 4 Keys to Getting Into and Staying in Flow.
2. Avoid Interruptions
One focused hour a day of uninterrupted creation time can turn almost anyone into a prolific creator, the key work being “uninterrupted.” When we get interrupted, it kicks us out of flow. In fact it can take up to 15 minutes to get back into flow after an interruption. Designate at least some small part of your day to be completely free of interruptions.
I got reminded by this when someome posted the following image on Facebook.
At one work place I complained about the public address system constantly saying “Attention, Attention please. Paging Joe Blow.” I finally convinced them about the affects of such interruptions. They went to individual pagers instead. This was in the days before cell phones, for those who can remember back that far.
I first read about the concept of being in the flow in the book Peopleware: Productive Projects and Teams.