Updated: Dec 23, 2018
Oncoming traffic will have priority - you must not cut across them if to do so would cause them to change speed or direction. If you can walk across then you can drive across!
Position just before your point of turn (marked with a + on the diagram below) when you need to stop for oncoming traffic or if the road you intend on turning into is blocked - this will make it easier to turn in the correct position. Positioning to the centre of the road will often leave enough room for cars to pass on your left as well as confirm your intentions to turn. Do not cut the corner.
Be prepared for the possibility of the other driver beckoning you on, most likely by flashing their headlights or waving you on. You must make sure it is safe to turn and they are beckoning you on and nobody else, especially where there are vehicles emerging onto the main road that they may be intended for.