As far as I know, there isn't any way for them to purchase in installments through direct checkout.
What I would do is have them pay through paypal. You would set up the item to allow 'other' payment type and then invoice them through paypal. You can set up all of the invoices at once with the different due dates on them.
Be sure to include the shipping cost on the invoice as well. Once all the payments are made then you can ship the item.
I would also include a clause in the listing something along the lines of "this item will be paid for in installments with the last payment being made by xxxxx date. If the payments are not completed by xxx date, all payments will be forfeited and the sale will be cancelled'.
That will protect you from someone who says they want the item but forgets to make the payment.
If you take a look at my policies, you will see the wording that I use for my layaways.