I need help with building an Event Aggregator

I have a situation where we have a legacy app that is several million lines of code.  It was written many years ago and did not follow any design principles at all and is a lot of pretty bad code.  We have taken a design centered approach to adding any new functionality to this app but we have ran into an issue. 

The winforms in the legacy code needs to open forms from the new and vice versa. This is causing circular reference issues of course, and I thought an Event Aggregator would work in this situation but I am not clear on how to implement it. 

I know its going to have to use the Observer but how I keep the solutions from referencing each other is still unclear to me. Any help would be greatly appreciated.  I have looked at Martin Fowlers explanation of the Event Aggregator and that helped but it was too vague to completely solve my issue.

Thanks.