That is how it works: Stand upright and look at your surroundings. Close your eyes and relax by breathing smoothly in and out. When you can feel your breath is smooth, open your eyes for only around a second and close them again. Do as your eyes are the aperture of a camera only opened for a moment. Now you try to "see" the taken picture in your mind's eye. Take your time; try to remember as many details as possible. Objects, shapes, colours. When you think you can't remember anything more, open your eyes and compare the picture in your mind's eye with the "Original".
Consider that this practice will take some time. You "pictures" will get more exact. To make it more difficult, you then can "photograph" a special situation at the oche --- for example where your dart will hit the double. Important is that you learn to eliminate everything around you, that you no longer bother with any disturbances.
"Our biggest weakness is to give up. The surest way to success is to always risk a new attempt." (Thomas Edison)
In competitive sport you not only need strength, stamina, coordination, speed and mobility, the sports needs to control the powers of concentration to be able to show his competitiveness precisely and fast in competition. The powers of concentration are a very important mental component of success.
Slow movements: balance shows whether you are really concentrated. Should you loose balance during this practice you didn´t manage to really concentrate. Look for a place you will not be disturbed. Place yourself on your stronger leg, the one you can stand longer on, and then move the other leg slowly to the front, to the side and back to your body. No problem should you shake, but you are not allowed the floor with this leg. Perfect it would be should you manage to stand motionless. It's easier, when you search a point on the floor and fix your glance to it all the time. You will not be distracted then. When you can master this, change to the other leg. And when you manage both your legs you can try it with your eyes closed. You'll be astonished how difficult that easy practice will suddenly be. Another possibility to make it more difficult is it to include "disturbances". Try to stand the pressure, it will lose in importance during a match.
The goal of this exercise is it to stay balanced despite the shift of your weight and despite all disturbances. You only can manage it when you are really concentrated. This is a good practice before an important match to find to yourself. Just try it!