How collect first, last and deposit over a period of time?

[Henley H.]

We have someone we'd like to rent to and are working with them re: there first, last and deposit payment. We're collecting the first and deposit upfront, but letting them pay the last month's rent on some type of payment system so they can do it over time. Any ideas for the best way to do this? Should we just collect a little extra each month until they have submitted the full last month's rent (and of course have the specific payments and what they're for outlined in the lease)? 

Any ideas about how to structure the last month's rent payment would be helpful- thanks!