It's not too difficult, just time consuming and expensive.
First, you need to get together your team of lawyers, accountants, underwriters, admin, etc. They will research the differing laws in each state in which you want to do business and draw up the policy. They'll find out how much money you need to keep in reserves to pay claims in each state, which will be in the millions. They'll also buy the reinsurance you'll need to pay any claims should you have a disaster and a large number of claims. This process can take a year or more.
They'll file the policies with the insurance department of each state. They'll probably need to make corrections and refile several times. because the states are picky. This process usually takes several years.
Once a state approves your policies you'll need to get together another team of sales, advertising, claims processing, customer service, and more admin. This can run several more months.
Then you can start selling. Remember, you'll need to keep that team of lawyers on the payroll and maybe even more lawyers on retainer.