Box breathing involves four basic steps, each lasting 4 seconds:
breathing in
holding the breath
breathing out
holding the breath
It takes some getting used to, so you’ll be distracted at first having to concentrate on your breathing.
I also like to meditate by emptying my mind and thinking about a giant, unlit room with a spotlight on a chair that I’m sitting in. Whenever my mind wanders to anywhere but there, I force myself to bring it back and only think about me sitting in that chair. I’m neither happy, sad, nor mad. I’m just existing in the chair; breathing and existing.
And if all else fails: don’t panic. Grab a towel and stick your thumb out. Maybe you’ll get lucky and catch a ride on a Vogon ship. And most importantly, “On no account allow a Vogon to read poetry at you.”
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.