The door of your home shuts firmly behind you, and suitcase in hand you step out -- after 15 years of marriage -- into the unfamiliar world of singles. s

How do you feel, when at mid-life your spouse asks for a divorce that thrusts you from the (sometimes, at least) cozy comforts of family and friends to, suddenly, a life alone?

"Shut out, and sorry for yourself," says writer Richard Schickel, who stood on that doorstep five years ago. "Terribly, terribly sorry for yourself."

You feel "a sense of loss," of "life's game plan being busted."

And, slightly, "unmanned," if you are a male who always thought it was his sex's perogative to make the break from the marriage bed.

Women, after decades of "bitter" experience, have created "a set of folkways, wisdom and just plain camaraderie," says Schickel, to ease them through the transition.

But for most males -- as for him -- it is an "awkward passage" of little support and learning to "sew on your own buttons."

Then there's the question of sex, which means recalling how to ask someone out for a date. "I never enjoyed it when I was 15," says Schickel, now 48.

But to Schickel, "the most poignant" break involves "the kids." Some men "seem to divorce the whole family. As a man, you really have to make an effort" to prevent this. He and his wife each have custody of a daughter, "and I spent a lot of money on airfare" keeping in touch.

A bachelor since his divorce, Schickel, longtime movie critic for Time magazine, has written an autobiographical guide for suddenly single men (or women), Singled Out (Viking, 115 pages, $8.95).

The transition period, he says, may last "nine months to a year," and "I don't think you fully settle down for 6 to 12 months after that.

"Then comes a moment -- it's not a big flash of realization -- that you have built a new life. You have some habits -- preferably not too firm -- that are congenial to you. You stop missing your former wife and are completely involved in your new life."

He negotiated this phase, he believes, perhaps more easily because of a basically independent nature. "I was lucky. I've always lived by my wits. aBeing a writer entails being alone.

"I'm a person who once presented with any situation tends to get busy on it quite quickly. I say, "This is a given. How do you live with this given?'

"I was extremely anxious to establish myself in a new role, to explore the entire foreign subculture" of the singles' life style.

"The biggest thing you have to learn is to pay attention to signs. Men and women send out signals of availability. Those signs tend to atrophy in marriage -- or they should, since you have no use for them."

So far, he's skipped the singles' bars. "They look just awful."

Don't, in that first year, he advises strongly, "retreat into self-pity." Make a date, if you can, "for the first night you're on your own" -- but "without any grandiose hopes or dreams." Just a dinner together to begin exploring "the manners and morals of mature courtship. You may not be ready for a huge and earth-moving romance.

"I'm certain I was the bore of the world, telling my sad story. She was extremely gracious. A little of that goes a long way."

Of course, he says, "No denying it -- you're not the really cute guy you once were." (Tall and with a slight paunch on a slender frame, he looks like a seasoned, graying prepie.)

But unless you've gone completely to lard, you have -- one hopes -- been able over the years to enhance your "charm, wit and style," which count more with women "than, say, a neat pair of pects." You probably are established in your profession, and -- even with alimony and child support -- have more money for wooing.

As a single, adds Schickel, "it seems to me you should acquire the capacity for self-sufficiency. It's going to make you a better partner. You're less of a nuisance."

Your passage toward renewed self-esteem may be helped along by a "transitional woman" (or man). "They exist for both sexes equally. She's a year or two further down the road. She can show you the ropes."

But "for a lot of people, the transition they make with a transitional woman is back into marriage" when they are particularly "lonely and depressed." wHe cautions against it:

"I think your second marriage is going to be better if you've really sampled the pleasures and perils of freedom, not to mention the infinite variety of womanhood that is presently available to the interested man."

Schickel has taken his advice, though he is certainly no advocate of bed-hopping. "That hobby is shortlived."

At one point, he strayed into "the valley of the wimps" by throwing himself repeatedly at the feet of a woman who wasn't interested. "It was a terrible experience, not like a disease, but close to it."

What he looks for now in a woman is "a fully grown-up human being. A lot of men my age enjoy playing mentor to someone much younger. When I say I don't date women under 30, that doesn't mean I don't think they are lovely. I just have trouble talking over that large a generation gap."

He also prefers "somebody who is fairly firm about who she is and what she wants in a relationship. I'm happiest with serious career women. They are responsible at least for a realm of their own happiness.

"I'm looking," he says, "for a relationship that I would enter into as permanent," given his philosophy that "permanency in a marriage is an illusion. pThe world really turns over rapidly.

"What I'm arguing is, enjoy this moment and get everything you can. There's no guarantee it will continue."

To other singled-outs, he appeals:

"Knowing what we've come to know, we surely are bound to behave toward one another with a certain kindness, civility, and tact, to ease one another's passage through this changeable and occasionally brutal world with consideration and gentleness."