We try to stay away from built-in furniture as much as possible. Its messy, it requires travel, and there's no shortage of cabinet shops set up for exclusively this type of work. I'm most at home in the shop but every once in a while its nice to get out and drill some holes in rich people's walls. (Forgive the photography, some sawdust found its way in behind the camera lens.)
Scribing is basically tracing the shape of the wall onto your work, and then cutting that line to ensure a clean fit regardless of the flatness or squareness of the wall.
To make this scribe I made the entire panel (left side of screen) removable, so that I could secure the cabinet in place, make the scribe, then fasten the panel back in place with screws through the inside of the cabinet. As you can see there's no gaps, thanks largely to a liberal amount of white caulk.
The two lengths of counter top were joined in place using dogbones, basically a fixed-nut and bolt setup that can be tightened from below. Then it was scribed, pulled off the wall and cut, and pushed back into place where it was fastened from below to the cabinets and cleats on the walls, here obstructed by a white modesty panel.
This positions the working surface of the sewing machine flush with the counter top for quilting. The front of the recessed box was left open to accommodate a knee-operated lever and the cord to the foot pedal. To the left, a powerstrip mounted upside down with its cord running along the underside of the counter and through a hole in the modesty panel to the obscured outlet behind it.
Looking out on most of San Francisco, the bay, and beyond that the East Bay.
The rest of San Francisco, more of the bay, and Oakland in the background. Man I have to get that sawdust out of my camera.