Measuring an Alcove That Is Not Square
Nine numbers before any material is ordered
One tape reading across the middle of a recess is worthless as a build dimension. The minimum honest record is nine numbers: width at the ceiling, at roughly 36 inches and at the floor, height at the left return, at centre and at the right return, and depth taken at both back corners and once in the middle. Write each one on a sketch at the place it was taken, not in a list, because a column of figures loses which end of the room it came from.
Typical results in a pre-war Yakima bungalow look like this: 61 and 3/16 at the top, 60 and 7/8 in the middle, 60 and 5/8 at the floor. The spread is more than half an inch. Nothing rectangular will slide into that opening without either a shave at the bottom or a wedge of light at the top.
Run both diagonals
Corner to corner in each direction tells you something the widths cannot. Equal diagonals mean the opening is at least a true parallelogram, and any error is a taper you can plane out. A difference of half an inch between them means the whole recess is racked, so a case with a square face frame will sit fine on one side and open up as a wedge on the other. Knowing that in advance lets the discrepancy be split across two edges, an eighth here and an eighth there, where nobody sees it, instead of arriving all at once in one corner.
Plumb and level are separate questions
Hold a six foot level or a laser line on each side wall and record how far out of plumb it is over that span, with the direction: leaning in at the top, or belling out at 40 inches where an old plaster patch stands proud. Then find the high point of the floor across the whole opening. The high point governs everything, because the base has to sit on it and be shimmed everywhere else. Set a laser line off that high point at a convenient height, say 30 inches, and measure down to the floor at 12 inch intervals. Three quarters of drop across a five foot opening is common in a house with a post and beam floor and a century of settlement behind it.
Story sticks beat transcription
Numbers get misread. A 1/16 mark gets copied as an 1/8, a 5 becomes a 6, and the error surfaces after the panel is cut. A story stick removes the arithmetic: take a length of straight 1x2, hold it in the actual opening, and mark the real positions directly on the wood. One stick for width carrying both wall faces and any obstruction such as a gas line or a switch box. One for height carrying the floor high point, the finished top, and the position of every shelf and rail. The stick then goes to the bench and the marks are transferred to the parts. No dimension is ever spoken, written or added up.
Build it small on purpose
The last step is the one that separates trade practice from a keen amateur. Take the narrowest width you recorded, in the example 60 and 5/8, and build the carcass narrower still, usually 3/8 to 1/2 inch under, at 60 and 1/4. The same applies to height under the lowest point of the ceiling. The case is now guaranteed to go into the hole with room to spare, which also means it can be squared and levelled on its own merits rather than being forced into the shape of the wall.
All the accumulated irregularity is then absorbed by parts that were designed to be cut in place: wide stiles at each side, a filler at the top, a plinth at the floor. Those pieces are fitted last, after the box is set and fixed, and each one is trimmed to the wall it touches. The visible outline follows the room exactly while the structure behind it stays true, which is the only way to get square doors and drawers in a space that has no square corner in it anywhere.