Enforceable Restrictions?

Well in some cases they were given that power by their legislatures. In others they get it for a limited time before they have to get permission from legislatures.
The solution is to ignore them and when it goes to court you contest and the injuct them from proceeding while you file criminal charges for violation of the constitution against every single person involved.

Most of time they didn't bother to get authority and several states shave lost in court because of that.

I may be wrong, but I think it is by executive order, just like the president. The governor is the state's chief executive.