How do you make nuclear reactors safe?

Firstly by design. The basic configuration and nuclear characteristics of the reactor should include things like having a negative temperature coefficient, so that if the temperature rises the reactivity decreases, giving an inherent degree of stability. The Chernobyl RBMK reactors had a positive coefficient, which didn't help when they lost control. The pressure vessel and the whole primary circuit needs to be made strong enough to withstand any predicted pressure surge, and to be made to the best possible metallurgical standards with all sections crack detected and all welds thoroughly examined. Then the systems analysts have to check through every aspect of design and draw up fault trees so that every possible fault is analysed and must be found to have a very low probability of failure, if necessary additional safety must be built in with additional control and instrumentation as required.

Then operation and maintenance is very important, adequate procedures must be drawn up, vetted by experienced engineers, and must be followed by well trained staff. Safety systems protecting the reactor must never be vetoed (as at Chernobyl).

The whole aspect of the above must be thoroughly vetted by the licensing authority, in the US this is the NRC, and they must have the power to withold a licence to operate if they think it necessary.

This is a very condensed explanation of a process that takes years of work to complete, and is in fact ongoing through the life of the plant.