Sold a house on contract and now buyer isn't making payments

[Holli Phillips]

This past August I sold an investment property to a lady and gave her 2 months before she had to start making payments.  She put $1200 down and have only received one monthly payment of $600.  As of Jan.1, she will owe $1800.  I told her that I needed her to move her stuff out and that I was going to be selling the house if she did not catch up by January 1st.  What would be the best and easiest way to go about getting her out?  I do not have keys to this house because she replaced doors when she bought it.