Outstretch your arm straight out to the side of you to indicate your intended turn. Ensure you make this indication before you edge out to the middle of the road in the case of a right turn, so other road users have plenty of notice of your intentions..
If you’re at the head of a large peloton, it can be helpful to raise your hand to just above shoulder height, and point in the direction of the upcoming turn.
This signal is used whenever there is a requirement to indicate, eg when turning or changing lanes. It is not normally used to move round parked cars or other hazards (see Oncoming Hazard Signal) unless you need to indicate to other road users that you are moving out into the traffic flow.