Mark the position of your hole onto the mortar with a marker.
Insert a sharp bit into a hammer drill's chuck, located at the tip of the drill. If the mortar is relatively fresh, a standard masonry bit will suffice. If the mortar is older and has had time to fully cure, use a percussion bit.
Plug the drill in, or insert a fully charged battery into a cordless model.
Put on a pair of goggles to protect your eyes.
Hold the drill against the mortar at the dot.
Squeeze the drill's trigger gently to establish the hole. This will help to prevent the drill from skipping, which could happen if you pull the trigger all the way at the start.
Increase the pressure on the trigger until you are drilling at a pace that you are comfortable with. Hammer drills combine a standard drilling action with the pounding action of a jackhammer, and it may take some time to get used to.
Stop drilling when you have reached the desired depth. Some hammer drills have a depth gauge that you can set before you start drilling, making it easy to tell when you reach the depth you want.