The Supplemental Draft started Wednesdayat 1 PM ET. If you want some background, here is a great article from the SB Nation mothership:
What happens when players are drafted? How are players drafted?
Supplemental draft picks work like an advance on future picks. When a player is chosen with a supplemental draft pick, that pick is subtracted from the team’s draft picks in the following year. For example, if a team were to spend a third-round pick on Beal in the supplemental draft, it would lose its third-round pick for the 2019 NFL Draft.