Once one understands the idea that the Due Process Clause not only tells the government how to do certain things (procedural due process) but also whether it can do certain things (substantive due process), one comprehends the hard part of substantive due process analysis. The rest is relatively straightforward. There are two basic steps.
First, the Court defines a limit on governmental action. The Court typically looks at cases from the individual’s perspective, finding in terms of a “fundamental right’” that government cannot violate.
If the court finds a “fundamental right,” it then essentially prohibits the government from violating that right. But, of course, there are no absolute prohibitions in constitutional law, so it is more accurate to say that the court uses a test, called “strict scrutiny,” that the government almost always fails. And it is easy to see why. In order to pass the “strict scrutiny” test, the government must prove two things: 1) that it has a compelling objective it is trying to achieve (something really, really important, like national security or protecting children’s health); and 2) that what it is doing is absolutely necessary to achieve that goal.