You need to send a certified letter, detailing the lease agreement late fee clause. Keep a copy for yourself, of course, give a date that the fee needs to be paid by and list any consequence that may be deemed appropriate.
What I do is I grace one or two late fees if the tenant is forthcoming with their circumstance. For instance one of my tenants was in the hospital and missed two weeks worth of work. Of course I would rather have a tenant than an empty so I let her pay over a few months. Since she is a good tenant and usually has not been late in the past I made an exception. Be careful with this though, you want to be fair in all your business. If you do it for one try to make sure you are reasonable and can do it for anyone else (like in 2-4 unit). You don't want any hint of favoritism ( that is an ugly horse to put down)
It's really up to you, is this a good tenant or someone who has been with you a while. Do they keep you informed? Do you have a payment on the property? Do you have money put away for these payments in case of vacancy? All of these questions factor in when dealing with tenants.
The day you get a tenant, better yet, the day before you get a tenant you have to decide what your standards are gonna be. What you can allow and what you can not. Some properties may be different than others, some you may be able to let slide a little and some you may not. Some tenants may be different than others in the same way as well.