When you move off from a downhill you are moving in the same direction as gravity so it can be a lot easier moving away downhill than a flat surface or an uphill road but equally can be rather dangerous too.
Firstly you need to assess how steep the downhill is. The steeper the downhill the higher the gear you’ll need. Prepare the car to move away by selecting the gear you’ll need, first gear if it’s barely downhill, second gear if it’s quite a bit downhill and third gear if it’s a very steep downhill.
Once you’ve chosen the correct gear, keep the clutch down and apply and hold the footbrake, remove the handbrake, have a good all round check and when it’s safe, signal and slowly ease of the footbrake. The car will start to move off, as you are picking up speed come completely off the footbrake then start to remove the clutch. Once you’ve removed the clutch completely you may need to start to brake very gently to control the speed. As the car is going downhill it can pick up a lot of speed especially if you are in a higher gear so you will need to gently brake from time to time in order to ensure that you have complete control of the vehicle.
General driving on a downhill is to change gears earlier than normal, this is quite the opposite from driving uphill where you have to change gears a lot later.
If you normally change gears on a flat surface at around 10mph, on a downhill you might even start to change from around 5mph because by the time you come off the gas and put the clutch down the car will start picking up speed as this is equivalent to putting it into neutral and putting it into neutral in a car going downhill can make it speed up quite significantly.
Barely any gas is needed on a car going downhill and a significant amount of braking will be needed to maintain complete control. If the downhill is very steep and you wish to keep your speed slow then you may be better off sticking to a lower gear as this will not let the car run away from you and avoid over using the clutch or placing it into neutral whilst in motion for as much as possible.