In Tennessee, our lease agreements and purchase agreement forms (TAR) are copyrighted, so we are forbidden to just pass them along to you (an unlicensed person) :o(
I have had several lease purchases to work in my 15 year career, but I have also had a few fall-outs. You must know what you are doing for them to be successful - I suggest you read my next paragraph below. First, you need a "lease agreement" with your tenant. In that lease agreement, you need to spell out time limits for conversion. At, the same setting, you will do a purchase agreement for the sale of your house and what happens if the tenant does not convert into a purchaser.
You have 3 options of writing this up -
1. Hire an attorney (this will be your most expensive route, but you know that it will be safe)
2. Enlist an agent that is savvy regarding lease purchases. Be prepared to pay from 1/2 of the first months rent to the full 1st month's rent as their brokerage, then their commission minus the upfront 1/2 rent when the lease converts into a sales agreement. You do not have to hire them as a property manager and it can be written where the monthly rental goes straight to you.
3. Go to an Office Supply Place, buy prewritten forms, hope they are approved for your perticular state's law, and then pray you don't do anything illegal by accident.