Doing a "knock-out" can be very time consuming. Unless you do it every day, plan on a long project. The magic wand will work sometimes, provided the background is all the same color and there isn't any of that color in the subject. Before you statr, make positive you have an original saved. For the large stuff, I use the eraser. Then gradually blow up the image using a smaller and smaller eraser till you get about every last pixel of the background. When I need knock-outs, I hire some one else to do them. It costs me on average about $30 a pic. Once you are finished, remember to save the clipping path.