Legal Question in Business Law in California

By issuing an order. If so, then you get to fight it out in court. Expensively.

I believe the way it's usually done is not for the Federal Trade Commission to "issue an order," but for it to go to court in the first place, to seek an injunction or similar order of the court. You can get some examples by putting selected search terms into Google such as FTC, block, order, sale, etc. and read the articles that come up.

I think you meant how does the FTC have the power to block the sale of a private company, rather than what their procedure is. The answer is the FTC is authorized to do that under the Taft Hartley Act, and regulations promulgated under that act. The Taft Hartley Act in turn is based on the Commerce Clause of the U.S. Constitution.