It's quite easy. First, take an old painting (buy one from a thrift store, use one from Grandma's basement, whatever you find that works)
My roommate and I chose this one from Salvation Army for about $6.00. We loved the colors and the frame.
Next, decide what you would like it to say. I chose lyrics from the popular song "Home" by Edward Sharpe and the Magnetic Zeros. Not only does it mean a lot to me due to listening to it with friends all summer, but also when I lived in Ireland last year my roommate sent it to me to cure a night of homesickness. It has become one of our songs ever since.
After all of the letters were adhered it just needed a few coats of gesso. I thought it turned out really well. My favorite part is the girls face peaking out between the E and T, definitely a great accidental moment.