The first step is to find a home with lease with option to purchase possibilities. Check with realtors in your area and even homeowners that have their homes for sale by owner. Even if a residential property is for sale, the owners may agree to a Lease/Option, just to have something in their hand. Also some of the online property search websites will have lease/options listed. You might also convert a rental property agreement to a lease/option.
Now you have located the home you want, the next step is the agreement. Whether you are the Tenant/Optioner, or the Landlord/Optionee, it is important that everything be in writing. In contract law, if it isn't in writing, it basically didn't happen. Remember, this is a residential rental or lease agreement with an addendum. One document. You can get forms off the internet, through business supply stores, or places like nolo.com. What is important here is what is in the agreement. All states have landlord tenant laws and those apply here.
The Lease Option addendum to your lease should, at a minimum, set out the following: length of option (a year is not uncommon); the provision that any breach of the lease will lead to termination of the option (that includes, of course, failure to pay rent on time; how the sales price will be computed if the option is exercise (e.g. fair market value as determined by an appraisal at the time the option is exercised); how soon before the option period ends does notice of exercise need to be made and how (e.g. 90 days before and in writing); that the property is sold "as is" at the time option is exercised; time line of closing after notice of exercise of option; and what is an isn't refundable in case the option is not exercised and the lease ends (security deposit for lease if governed by landlord tenant law and must be refunded with certain exceptions but the option fee is negotiable.)
More about the option fee or rental payment credit. Most parties to a residential property lease with option to purchase agreement reach an agreement on whether any of the rent will go toward a downpayment on the house. This is negotiable. What usually isn't negotiable, is that the tenant put down an option fee in exchange for the landlord, or owner, of the property keeping the house off of the market for the length of the option. The amount is negotiable and is usually nonrefundable if the option is not exercised. It is often, however, creditable for when the option on the home is exercised.