I've created a junction object called "OCP"

On object "OCP" I've created two Master-detail relationships, one that points to my custom object "Company Products" and one that points to the standard object "Opportunity". My custom object "Company Products" also has a Master-detail relationship with the standard object "Accounts".

"Company Products" <---Master-detail---- "OCP" ----Master-detail---> "Opportunity"

My sales reps need to be able to create or select "Company Products" records from the Accounts page layout (which I'm doing with standard functionality due to the master-detail relationship Company Products has with Accounts) and from the Opportunity page layout (this is where I need help).

Currently when I add the Company Products related list to the Opportunity page layout I can add all of the fields, but the "New" button is trying to create a New Junction Object record and I need this button to allow our reps to create new, or select existing Company Products records for an Opportunity.

I'm sure I do this by creating a custom button, however the custom button stuff that I have read really does my head in (I'm still a fairly new SFDC admin and am in the early parts of the learning curve).

Could someone explicitly walk me through what I need to do here? I've done extensive google searching to try to find an example, but I couldn't find anything directly applicable to what I'm trying to do (my google skills failed me!).

Thank you!