Working with the Composer takes some practice. Besides changing the aperture ring, you have to get the hang of shifting the front part of the lens element appropriately. The front part of the element mounts to the rear end via a ball like connection. You can then pivot the front part all through the image frame in order to place the focus point where you want.
Since you’re relying on the aperture ring to control the amount of selective focus you get from the Composer, you have to work on controlling exposure in other ways. Shutter speed and ISO usually provide enough control, but you can also use flash if need be. It’s not possible to mount a filter on the front of the Composer, using neutral density filters isn’t an option. Many cameras (but not all) can use aperture priority automation for auto exposure with the Composer.
Obviously focusing is done manually. This is another area where the photographer needs some practice, since it can be hard to focus precisely with the Composer.