Here is a TurboTax article that will walk you through the process of entering your RSU's and the adjusted cost basis: RSU's and video on how to enter them.
The amount of the RSU that was included in your W2 Compensation (generally this amount is also indicated in Box 14 of your W2) is your basis in the total units which were included in Box 1. Any additional cost when they were sold, would be on your 1099-B as cost basis and should be included in the Total Basis on Schedule D.
Edited (KarenM90 2/18/20 6.56 pm pst)
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