Creating a Connectable Web Part {MSDN]

There are 11 high-level steps that you must complete to implement a connection interface for your Web Part:

Create the interface class.
Declare events.
Override the EnsureInterfaces method, and then call the RegisterInterface method.
Override the CanRunAt method.
Override the PartCommunicationConnect method.
Override the PartCommunicationInit method.
Override the PartCommunicationMain method.
Override the GetInitEventArgs method.
Implement the interface event handlers.
Override the RenderWebPart method.
Implement supporting methods.