For a more complete  integration guide, see the Google Analytics ROI Integration Guide. This is step 3. 

1. In Google Analytics, under Admin > Property Settings, you will see your Tracking Id and Default URL. This is the Tracking Id for the website you want to track with ROI. Make sure the Default View is set to All Web Site Data.

2. Go to Google Analytics > Admin  > Custom Definitions > Custom Dimensions (see the picture below for navigation).

3. Add four new Custom Dimensions with scope “Hit”. Add a fifth Custom Dimension titled “ROIID” with scope “User” (See below). 

NOTE: The dimension titled "ROIID" with the scope "User" needs to be in the Index 5 row. Therefore, if your Google Analytics already have Custom Dimensions created, then you need to create enough custom dimensions so ROIID is in the Index 5 row. For example, if Custom Dimension 1 is already created, then create Custom Dimension 2, 3, and 4 with scope "Hit". Then Create ROIID in Index Row 5 with the scope "User".