To enter "adjusted cost basis" do I just add up the adjusted cost basis for the relevant transactions out of the supplemental form?

Yes. You would enter the cost basis. Add the amount included as part of your wages on  your W-2 and any additional discount amount you might have paid when received, if applicable. Each vesting date is important and the stock sold will be considered first in first out (FIFO) unless you selected specific shares (uncommon).

See the link below for more information about the sale of RSUs.

• How to Report RSUs or Stock Grants On Your Tax Return 
  • Excerpt: After you take ownership and pay the income tax on the fair value of your stock, you treat the stock for taxes the same as if you bought the stock on the open market. Here are the different ways you can be taxed:
    • If you sell the stock at a higher price than its fair value at the time of vesting, you'll have a capital gain.
    • If you hold the stock for one year or less, your gain will be short term, and you'll owe ordinary income tax on it.
    • If you hold the stock for more than a year, your gain will be long term, meaning you'll pay tax at the more favorable capital gains rate.