THIS LADY who runs a boarding house which she calls "a house of destitution, for students who're hard up, money-wise," once let a nascent doctor change a washer on her bathroom sink.

"He wanted to prove he could do it like a regular guy," she says. "Profesional kids - med students, law students - they're [WORD ILLEGIBLE] that. He probably thought [WORD ILLEGIBLE] something off his rent. "Ha - he thinks," I said.

"He got the old washer off and the new one on," she admits, shaking her head No in doleful qualification, "but he forgot to turn the water off before he loosened the faucet, and it was all over." She slaps her cheek.

"Soaked the wiring under the floor so bad I had to get an electrician. But when he put the knob back on, he tightened it" (She rows the air with an ima ginary wrench) "so much that he stripped the threads, and you could turn it" (she spins invisible faucets) "till lord knows when, and not get anywhere.

"I called a plumber, and he charged me 46 bucks, which you can believe went on 'Doc's' rent.

A lot of us have tried to do something useful and simple around the house, like change a washer on a faucet - and failed. And probably as many "regular guys" fail as "professional folks" do, but the regulars keep quiet about it, while the pros claim it as proof of loftier talents.

("How do you expact me to fix a sink, dagnabbit!" growled the med student. "I'm used to working with my mind, not my hands.")

"I could've done it myself," chuckles his landlady. "In New York, where I grew up, fixing a faucet was the next thing a girl learned to do when she was old enough to cook eggs."

She led us to the kitchen and pointed to the cold water tap. Bumper-to-bumper drops fled the spigot. Obviously set-up.

"There're two ways that faucets leak: from the handle or from the spout. If it's the handle, it's usually because the packing nut under the handle is loose, or because the washer under the nut is bad. First, try tightening the nut, but not too hard, or you'll ruin internal parts and make the leak worse. (It's a good idea to wrap the wrench in tape or cloth so you don't scratch the chrome.)

"If tightening the nut doesn't stop the leak, shut off the house's water supply. Now, see the cap on top of the handle? You can spot it by the 'H' or 'c' that's usually printed on it. Pry the cap off with a small screwdriver. Now you see a screw running through a center of the handle. Remove it, lift off the handle, unscrew the packing nut, lift it off, and there's the packing washer. If it looks worn, replace it, and put everything back together. That should stop the leak. If it doesn't, you might need to replace the fixture.

"If the faucet leaks from the spout instead of the handle, another washer is probably to blame. Take off the cap, handle and packing nut, then put the handle back on without the packing nut. Now, turn the handle until the stem assembly comes free of the faucet.

"At the bottom of the stem, you'll see the seat washer. If it's worn, replace it with one that can be used for both hot and cold water. If that doesn't take care of the leak, you probably need a new valve seat, which is a slightly different matter that you might want to leave to the plumber.

"Some faucets are built differently. But you can figure most of them out by common sense. Remember one thing: the soft metal (like brass) used in fixtures can be ruined by too much pressure on the wrench or screwdriver, so if you can't keep your temper, call a plumber."

Her advice aside, you might want to check out these BOOKS

Complete Home Plumbing & Heating Handbook by Jeannette T. Adams. (New York: Arco Publishing Co., Inc., 1977.) A fix-it manual written with an eye to esthetics. Includes plans for remodeling bathrooms, kitchens and heating systems, with discussions of design, costs, use of space, tools and materials. Heavily illustrated.

Basic Home Repairs by Walter Ian Fischman, et al. (New York: Grosset & Dunlap, 1977.) Illustrated sections on plumbing, carpentry and electrical repairs. Less technical than Adams' book.

The New York Times Guide to Home Repairs Without a Man by Bernard Gladstone. (New York: Quadrangle/The New York Times Book Co., 1973.) A little more literary than the others. Covers basics of plumbing, painting and electrical repairs. Illustrated.

How To Fix It by Ann Singerie. (New York: Doubleday & Co.,Inc. 1974.) Simple and humorous.