The same way we would privatize anything else. For police, you subscribe to a security service. When you are subscribed, you can call them for protection or to report crimes. This model incentivizes them to provide prompt, high quality service. Like many private corporations do now, they probably will also provide some basic services to everyone for free, to promote their brand and build good will. This will align incentives between the customers, security, and shareholders. For those with higher security needs, they could subscribe to something like a gold tier which would entitle them to faster service, more thorough investigations, etc. They could also subscribe to multiple companies, so they can get redundancy.

As for fire, same idea. Fire stations would be privatized. You can either decide to pay out of pocket for incidents as needed, or you can subscribe. You would pay a monthly fee for coverage, probably with a deductible to prevent over use of services. The companies that run the fire stations can also set up their own network of interoperability, so if your car catches on fire somewhere else, the local fire department could be in your network, and you would only need to pay the standard deductible. You could also get something like a neighbor add-on to your insurance, so that if your neighbor's house catches on fire, the fire department will come help as much as is needed to prevent it from spreading to your house. Again, this would incentive fire departments to have prompt, high quality service. The competition would drive down costs. This would also create an investment opportunity for shareholders, so they could invest in fire service companies. You could bundle products with things like homeowner's insurance. There would be a ton of business and efficiency opportunities.

Most importantly, a privatized fire and police would force more personal responsibility and stop with all this socialism nonsense. People make lots of risky decisions because they know they can just call the police if they have problems. Now they would need to consider their actions more carefully. It also allows people to judge their own levels of risk. Most homes don't burn down. If you're careful, you can decide you don't need fire insurance. If you are an easy-going person who generally stays home after it gets dark, then you can choose to not buy a police subscription, or maybe some kind of light subscription only for emergencies. This would mean that instead of everyone paying the same amount, with some people using the services way more than others, now everyone pays for what they need, and takes more personal responsibility.