To see how to add arrangements when creating a Deal, please check the article Deals - add a deal

If you have to edit an existing Deal and add more arrangements to it, go to Deals list and open the deal to be edited. In Deal page, there is the Arrangements section :

Click ADD NEW and proceed the same way as in create deal process, meaning that is mandatory to select begin and end date for searching then select the product you need to add as a new arrangement to the existing Deal.