If you don't want to make your own lox, you can substitute store-bought smoked salmon. However, you must cut it properly. Place 4 ounces (113 g) of smoked salmon on a cutting board, and position your knife against the fish at a slight angle in a downward motion. Any sharp knife with a long, non-serrated blade works well for cutting the salmon. However, you can also purchase a special salmon knife that's designed specifically for cutting smoked salmon thinly. Make sure that your knife is as sharp as possible for the cleanest cuts. To create the appropriate texture for lox, it's important to slice it as thin as possible. When you insert the knife's blade into the fish, keep it as shallow as you can so you can create a paper-thin slice. Once you've insert the knife into the salmon, move the blade back and forth in a smooth motion until you've cut a slice. You shouldn't have to push too hard to cut the fish, so if you feel resistance, you may want to sharpen your blade. Use the length of the knife's blade to make a long, sweeping back and forth motion for the cleanest cuts. After you've cut the first slice, repeat the cutting process until you've sliced all of the salmon. Be sure to leave the skin behind as you cut, and remove any black meat that you come across.

Summary:
Slide the knife over the smoked salmon at an angle. Insert a knife into the smoked salmon shallowly. Use a back and forth motion with the knife to slice the salmon. Continue slicing until all of the salmon is cut.