Part of it is just working the muscle. I've basically killed everything in my phone that notifies except for text messages and phone calls. And even those go quiet when I set it to Do Not Disturb.

From there it's about training. It's almost like meditation where the more you do it, the longer your stretches of focus can become and the more quickly you can click your brain into "reading more." To ease back in I found graphic novels were a great start. They accustom you to sitting still and getting absorbed into things.