Just position the cloth directly above the chair, and allow it to fall under the influence of gravity - nothing needs to be moved during your simulation, but do make sure you have allowed enough time for things to happen. What does actually happen when you run the simulation? Cloth doesn't move, or not enough? - try adding more frames, or reduce the cloth density so it will fall faster. Cloth falls through chair? - check your "collide against" settings. Something else? Post a screen shot of the results, and someone will come up with an answer!