You would first write up a purchase and sale agreement. Which may require contacting an agent an maybe negotiating a set fee that you would pay he or she. The next thing you would need to do is come up with a down payment that works for you and your tenants. The next thing you would do is contact a real estate attorney and have them to do the rest. You can put special stipulations in the deed that would keep you from harm if they default or any problems come up. There will be closing cost involved in this process.