You put the ends of the cable into C and crimp C around the cable. Don't use to much force, the little tabs can break.
Then you plug those silver things into the golden things.
If D is a soft tube, as in heat shrink, put it over the cable before attaching C, when you have plugged the stuff together slide it over the joint, heat it slightly, it shrinks and protects the connection.
Make sure you have all the cables where you want them on the bike and in the length you need them before you do this.
I think B are things that go around the cable into your back light somehow.