There are several cut and dried ways to do this and it can be customized to suit the situation.
Here are the basics:
Option1: The tenant/buyer makes a partial down-payment at the start of the lease and a portion of the monthly rent is put towards the down-payment.
Option 2: There is a set base rent and the tenant/buyer pays a set additional amount each month towards the down-payment.
Option 3: The owner is financing the purchase, and all, or part of the rent is applied directly to the purchase price; there is usually set set time period in which the renter/buyer will obtain financing for the balance.
Option 4: The renter/buyer pays an additional amount over the monthly set rent, and this amount and their security deposit are applied as the down-payment.
Diana please keep in mind that if an owner is willing to enter the rent to own process that they are usually open to many different options and the reason why they don't specify any particulars, is that they don't want to limit the market base for their property. If you are considering entering into a lease to own property, let them know what will work for your budget, and negotiate a custom deal that will work for both you and the seller.
I agree with Aimee below. Hire a good real estate attorney to draft or look over any lease/purchase agreement you man enter into. There's too much at risk to leave any loopholes or T's uncrossed. And as Patricia noted, the possibilities with Lease/Options are endless.
These can be tricky, espcially in such a fluctuation marketplace. First, you should have a good real estate attorney. I can recommend a couple of attorney I use a lot, if you'd like.
But, the bottom line is that the buyer and seller agree to price now, enter into a sales contract by which some of the montly rent would go towards the purchase price at the end of the term. If the buyer does not want to purchase at the end of the term then all the "rent" monies are just that, rent, and are kept by the seller/landlord. In this fluctuating market you may want to have some terms be that the price is negotiated at the end of the term, but that's a risk for both the potential buyer & seller. The price could go up but it could also come down.