The first thing that you need to do when creating your own Custom Report is to select the different widgets that you would like displayed. As you might already know, you can add the widgets that suit you best by choosing from the different widget categories available and then explore their amazing potential.
The Website Analytics Category includes information regarding statistics coming from your website. One of the widgets included is named "Cart Conversion Rate for the Entire Account". This widget provides you with a summary of all the carts and purchases for the entire account. Please note that said widget can not be filtered since it has been set according to the entire account.
1. Select the Cart Conversion Rate for the Entire Account widget by clicking on the Add button and then tap on Save. 
Please keep in mind that there are several filters to choose from that can be applied to the corresponding widgets. Furthermore, you can always export the data of each widget in order to have them at your disposal at all times.