Images by Peter Jackson Dip.DI, DSA.ADI(Car)
How to use a Box Junction

Box junctions make it possible to keep the flow of traffic moving and therefore keep the junction clear, by preventing traffic from stopping in the path of crossing traffic.
Use the Mirror, Signal, Manoeuvre routine (MSM) early and this will give you time to assess the junction before you get there.
Our diagram on the left shows a box junction at traffic lights. You should enter the box junction if you wish to turn right and your path is blocked by oncoming traffic.
It is OK for you to wait on the yellow criss-cross providing your exit road is clear.
If you wait outside the box in these circumstances, you could fail your driving test.
When the way is clear move away smartly and do not cause an obstruction.
Be careful when queuing inside the box junction as you may not have time to clear the junction before the traffic lights change.

The diagram on the right shows When you should not enter the box junction

You MUST NOT enter the box until your exit road or lane is clear.
Because there is a line of traffic already in the road on the right, there is no room for you to join this queue without blocking the flow of traffic shown by the yellow cars.
In these circumstances you should not enter the box.
If you enter the box in these circumstances, you could fail your driving test.

Box junctions shouldn't be feared. If anything they make life easier. Like everything else when learning to drive, keep practising.