How to create custom events in lightning components of Salesforce?

Create a custom component event using the <aura:event> tag in a .evt resource. Events can contain attributes that can be set before the event is fired and read when the event is handled.
Use type="COMPONENT" in the <aura:event> tag for a component event. For example, this c:compEvent component event has one attribute with a name of message.
<!--c:compEvent-->
<aura:event type="COMPONENT">
<!-- Add aura:attribute tags to define event shape.
One sample attribute here. -->
<aura:attribute name="message" type="String"/>
</aura:event>
The component that fires an event can set the event’s data. To set the attribute values, call event.setParam() or event.setParams(). A parameter name set in the event must match the name attribute of an <aura:attribute> in the event. For example, if you fire c:compEvent, you could use:
event.setParam("message", "event message here");
The component that handles an event can retrieve the event data. To retrieve the attribute value in this event, call event.getParam("message") in the handler’s client-side controller.