Recognize that your most focused mindstate is first thing in the morning. Do tasks that require the longest attention span right then. Don't turn your phone on until they're done.

Generally don't keep your phone in your pocket, put it in another room on silent. Disable lockscreen notifications for all apps but the essentials. Do the same to your computer. No screen time 2 hours before bed.

Do long cardio workouts outside.

Recognize that it's OK to be bored, you don't need to fill every gap by looking at your phone. It's OK just to let your mind wander around.

You give up looking at screens in your free time, that's the only real answer. It's sucks, it's hard, and I've had no success at it, but that seems to be the key.

Curb usage of attention-seeking technologies. Shift your habits. Start by acknowledging the problem and its sources. Distance yourself from them psychologically, then physically.

You just pay attention longer. It takes effort but that's all you really need to do. For example, go observe and pay attention to a plant for 45min. Think of it like a workout, it's uncomfortable and a PITA but that's all there is to it.