Need Help On A Flip Possibility

I have this opportunity to buy this house ... but I want to do a lease purchase over a 6 month time period. They are 3 months behind in their payments. I am willing to get them up to date with payments as a down payment for the lease purchase deal. But want to flip it. I can put the house under contract for $170,000 and I know I can sell it for around $220,000. Can someone give me the details on getting this deal done properly? Thanks so much. I need to get this deal signed by 12/20