Having just passed my driving test yesterday I can at least tell you the official way.
If you can, you try to keep the car rolling, it saves on fuel and its strongly emphasised as part of making driving ‘greener’ - this apparently reduces fuel consumption.
This also has the effect of forcing you to plan much further ahead, you slow down earlier to try maintain some momentum so you have to predict what is going on in front.
This also reduces your use of brakes, which dispose of kinetic enrgy as heat, which is apparently not very green.
You do not use the gears for slowing down, you simply look further ahead and come off the gas sooner.
You are alowed to coast in neutral, but only if you are already applying the brakes, and only if its to roll up to stationary vehicles where you are just making up a gap in a queue of traffic.
You are expected to remain in the highest gear that the car can readily cope with, and when you change gear you are now allowed in the test (you are actually expected to demonstrate it) to change through a couple of gears at a time, say from 4th approaching a turn, slow down and drop into second once you have braked down to the correct speed.
This is known as ‘block changing’, you tend to use it more at stops and traffic lights, where you approach in one gear such as third or fourth, stop and simulatneously go into neutral, then switch either to neutral or the gear to intend to resume moving, usually first, but it could be second on a downhill start.
Using gears to slow down instead of brakes in the UK will be classed as a minor fault, collect enough of these and you fail your test, do it hard and it may be classed as a serious fault and fail you in one go.