Delinquent tenants are a landlord's fear. You can prepare all that you can, but if that rent stops coming in, something has to be done and done quickly. The first step is to reach out to the individual and try to contact them, requesting the rent that is owed to you as their landlord. There may be times that when you do get a hold of them you find out that they simply can't pay the money they owe you. That's when the decision needs to be made. Sometimes it might be more prudent to give them the option to simply leave, but other times you'll go through the eviction process. To do this you'll need a Pay Rent or Quit Notice. This details out the amount of money the tenant owes and the time that they have to pay it before you move to have them evicted. Details of this process may change slightly from state-to-state, so make sure to check your local laws to make sure that you're following the law carefully. It's a good practice to send one notice through the mail, but also to have someone tape a copy to their front door. Often this will bring about a call from the tenant to try to sort things out.
If they don't contact you, that's when you'll need to move on to the eviction process. Immediate action is key at this point because if they've ignored all your calls and other forms of communication, they likely will not respond to you at this point. To read more on how to use a Pay Rent or Quit Notice, take a look at that here