Photo The living room of the designer Bennett Leifer’s home in New York. Credit Jane Beiles for The New York Times

How do interior designers make the choices they do?

That’s the question I ask every time I step into an artfully decorated room or look through shelter magazines in admiration. Why were those framed drawings arranged above that Art Deco table? Why were the floorboards in the loft apartment stained black? Why, out of thousands of sofas, go with a boxy seven-footer upholstered in deep green velvet?

I never have a clue.

Despite my love for interior design and architecture, I don’t understand space and composition, and lack the ability to put a room together with confidence. Decisions are made arbitrarily, then regretted.

One of my favorite design books is “Jacques Grange: Interiors” by Pierre Passebon, which showcases the work of the renowned French decorator. Mr. Grange’s layered, bohemian rooms for clients like Yves Saint Laurent, filled with rare and beautiful objects, art and lots of chairs, are an ideal for me, and I must have studied the images a thousand times. But if the book is a kind of bible, it’s an inscrutable one, composed in a visual language I can’t translate.

What I often wonder about decorators like Mr. Grange is whether they are following an established set of rules — about space, color, proportion, materials, and so on — and experimenting within a framework of dos and don’ts. If there are rules, then an amateur like me can learn them.

Or is decorating a series of subjective decisions made by people with innate good taste — a make-it-up-as-you-go profession? In which case, I will never master it.

To help me better understand, I called Bennett Leifer, a young, New York-based designer whose work has been featured in Elle Décor and House Beautiful.

Although Mr. Leifer, 35, didn’t formally study design, he did serve a long apprenticeship in the field, working for Ralph Lauren in the brand’s store development, for the architect Robert A.M. Stern and for Juan Pablo Molyneux, the high-end residential decorator. When he felt he was “cooked enough, ” Mr. Leifer said, he started his own interiors firm, in 2013.

“For me, what I do is so emotional, ” Mr. Leifer explained. “The right way to do things is what sets up a backdrop for how a client is going to live. It’s not, for me, about color theory.”

But, he added, “There are rules you learn along the way. I learned a lot about proportions and dimensions.”

In talking with Mr. Leifer, I realized what I really wanted was to see a room through a designer’s eyes. I wanted him to break down each aesthetic decision, and explain why he’d made it.

Mr. Leifer was game, and we decided to deconstruct the living room of his Manhattan apartment, which was sufficiently stylish to be featured in Architectural Digest. I also asked him to view a space unfamiliar to him — the living room of my apartment in Brooklyn Heights, which is not exactly A.D. worthy. Indeed, if there were decorating rules, certainly I’d broken a few.

Listening to Mr. Leifer explain his work and critique mine turned out to be the revelation I’d hoped for. Sometimes he taught me a rule I didn’t know, sometimes he operated on instinct, but the reasons were always clear and logical. Surprisingly, so many of Mr. Leifer’s decisions were not guided by beauty but common sense and functionality. For me, it was interior design demystified.

Here’s what I learned at Mr. Leifer’s apartment

Casement windows Mr. Leifer restored and refinished the architectural details — the windows, floors, fireplace and walls — and used them as the “envelope” that establishes the colors for his décor. “So it’s black, dark brown, white and neutral, ” he said. “That was my main palette.”

Ceiling beams Mr. Leifer followed the lines of the ceiling beams to divide the room into three rectangular areas. He could then address his various desired functions within each. The middle rectangle is centered by the fireplace, and forms the heart of the room.

Couch, side table Mr. Leifer bought a sofa and occasional chair that are roughly the same cushion height, with backs and arms that are also within approximate height, because “you want the proportions of the furniture that go into a group to relate, ” he said. He believes a sofa should be 7 to 9 feet (his is 7) because 10 is too large unless it’s for a media room and 6 or less feels like a secondary sofa.

Television Where to put the TV is always a vexing issue. Mr. Leifer turned the room’s least prominent corner into a utilitarian space. “Just make this easy and functional and keep the beauty elsewhere, ” he said.

Photo Credit Jane Beiles for The New York Times

Bookcase Mr. Leifer arranged his bookcase by following two personal rules: He sorted the books according to size and put the less important books and storage binders on the lower shelves out of sight.

Photo Credit Jane Beiles for The New York Times

Mantel As someone who buys what he likes and figures out where it will go later, I was struck by Mr. Leifer’s discipline. Every piece adhered to his established criteria, be it color, function, budget or, as with the objects on the mantel, his “very elementary 1 to 10 scale”: Don’t buy something unless it’s a 10. You reduce clutter that way, and surround yourself with things you truly love.

Now, to my apartment

Photo Credit Jane Beiles for The New York Times

This is my living room after Mr. Leifer rearranged things.

Table Before, the table was shoved against the wall near the plants. “Things need to be put in places where they’re actually usable, ” the designer said. “There should be clearance to walk around the table, to pull the chairs out.” He suggests a minimum 30 inches of clearance between furnishings.

Painting As a general rule, art should be hung at “5-foot center” — meaning the center of the work is 60 inches off the floor, Mr. Leifer said, demonstrating by lowering a painting that I had placed too high. “Some people who are more sophisticated than I am break the rules, ” he noted.