After cutting or shaving off the small knob on the BL-08 battery you insert it into the SL-C700. It's a tight enough fit that if you have purchased the battery and not the kit (i.e. you don't have the extra cover) it will stay securely in place. It desn't look great this way, and rocks a little when placed on a flat surface, but is serviceable. The first picture above shows the SL-C700 with an inserted BL-08 battery alongside the plastic cover.
The second picture above shows a small plastic piece that you get along with the battery cover. This is inserted in the small hole in the case that is right next to the headphone jack (as shown in the third picture). It is a very snug fit - once inserted it is very difficult to remove. Once this is done you slide the black cover on the bottom (fourth and fifth pictures) until it fits tightly. (Note: you have to have the battery latch in the 'open' position while you do this). On the battery side the lugs on the black cover fit into the case, while on the other end the cover is held in place by the small piece inserted earlier (see the sixth picture above). Even though there is nothing holding it down on the fourth corner, once assembled it is very tight and feels very secure.
The last three pictures above show the "expanded" SL-C700 from various angles. It is slightly heavier and a little bigger, but when carrying and using it, it did not seem very significant. What is significant, however, is the improved battery life. I used to use the SL-C700 at the second to lowest brightness setting, getting a great screen and saving the battery... now I keep it cranked up a notch or two higher...