Once the woodworking is finished, you'll need to choose your finish. If you're not going to paint or stain the finished product, most of this step can be disregarded.
You'll want to use a nail set to set the finish nails beneath the surface. Then, fill the holes with a wood putty or nail filler.
Next, sand all surfaces smooth using a sander with an increasing vareity of grits. When the sanding is finished, remove any sawdust with a tack cloth and paint or stain as desired.
Once you've achieved your desired finish, it's time to mount the cabinet. To do so, simply lift the finished unit into place and set the French Cleat (inside the back of the assembly) onto the wall-mounted wedge. Check to see that the unit's back is flush with the wall and securely in place before letting go.
Finally, check the box to make sure it is mounted level. If so, you're free to fill the finished cupboard as desired, as this project is completed.