If you struggle to visualise, just focus on your breathing. When you notice you are distracted bring it back to your breathing. Acknowledge where your mind drifted but try not to dwell on it as you focus back.

There are also apps available to help with breathing exercises. I had one that showed a balloon inflating and deflating. I found it easier to focus on that at first.

Counting while you box breath can also make it easier. The count can be whatever you’re comfortable with. 5 5 5 5 works well for me.